Add -Werror=unused-but-set-variable to our default compiler flags and fix
```
epan/dissectors/packet-dcerpc-frsrpc.c:709:10: error: variable 'nb_chunk' set but not used [-Werror,-Wunused-but-set-variable]
guint32 nb_chunk = 0;
^
```
```
epan/dissectors/packet-dcom-oxid.c:175:13: error: variable 'u32ItemIdx' set but not used [-Werror,-Wunused-but-set-variable]
guint32 u32ItemIdx;
^
```
```
epan/dissectors/packet-l2tp.c:1775:104: error: parameter 'ccid' set but not used [-Werror,-Wunused-but-set-parameter]
static int dissect_l2tp_ericsson_avps(tvbuff_t *tvb, packet_info *pinfo _U_, proto_tree *tree, guint32 ccid)
^
```
```
epan/dissectors/packet-ldp.c:1922:19: error: variable 'ix' set but not used [-Werror,-Wunused-but-set-variable]
guint8 ix;
^
```
```
epan/dissectors/packet-nas_5gs.c:4757:14: error: variable 'curr_len' set but not used [-Werror,-Wunused-but-set-variable]
guint i, curr_len;
^
```
```
epan/dissectors/packet-per.c:1769:6: error: variable 'extension_addition_entries' set but not used [-Werror,-Wunused-but-set-variable]
int extension_addition_entries;
^
```
```
epan/dissectors/packet-rtitcp.c:618:11: error: variable 'messages_count' set but not used [-Werror,-Wunused-but-set-variable]
guint messages_count, offset;
^
```
```
epan/dissectors/packet-tcp.c:2130:9: error: variable 'ackcount' set but not used [-Werror,-Wunused-but-set-variable]
int ackcount;
^
epan/dissectors/packet-tcp.c:3317:12: error: variable 'nbOptionsChanged' set but not used [-Werror,-Wunused-but-set-variable]
guint8 nbOptionsChanged = 0;
^
```
```
epan/dissectors/packet-zbee-zcl-se.c:11802:15: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
for (gint i = 0; tvb_reported_length_remaining(tvb, *offset) >= 5; i++) {
^
```
```
ui/iface_lists.c:142:23: error: variable 'linktype_count' set but not used [-Werror,-Wunused-but-set-variable]
gint linktype_count;
^
```
```
ui/voip_calls.c:456:15: error: variable 'item_num' set but not used [-Werror,-Wunused-but-set-variable]
guint item_num;
^
```
```
file.c:572:17: error: variable 'count' set but not used [-Werror,-Wunused-but-set-variable]
guint32 count = 0;
^
```
```
file.c:3667:24: warning: cast from 'const unsigned char *' to 'unsigned char *' drops const qualifier [-Wcast-qual]
pd = (guint8 *)ws_mempbrk_exec(pd, buf_end - pd, pattern, &c_char);
^
```
```
ui/qt/io_graph_dialog.cpp:1932:60: error: variable 'mavg_right' set but not used [-Werror,-Wunused-but-set-variable]
unsigned int mavg_in_average_count = 0, mavg_left = 0, mavg_right = 0;
^
```
```
ui/qt/stats_tree_dialog.cpp:166:9: error: variable 'node_count' set but not used [-Werror,-Wunused-but-set-variable]
int node_count = 0;
^
```
```
ui/qt/models/profile_model.cpp:1142:13: error: variable 'entryCount' set but not used [-Werror,-Wunused-but-set-variable]
int entryCount = 0;
^
```
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