srsue/rrctl: reserve a range for RRCTL protocol extensions

The idea is to group protocol extensions and have one message
type (with 4 variations: Req/Ind/Cnf/Err) per group.  Similar
to the stack and heap allocation model in Linux, new entries
shall be added to the 'RRCTL_MsgType' list as follows:

  - regular messages - from bottom to the end,
  - extension groups - from the end towards the bottom

The mask '11xxxx'B gives us 16 unique messages, one is reserved.

srsue/hdr/stack/upper/rrctl.h

@@ -40,6 +40,7 @@ namespace proto {
     RRCTL_CONN_RELEASE,
     RRCTL_PAGING,
     RRCTL_PARAM,
+    /* RRCTL protocol extensions (0b11xxxx, up to 15 groups) follow */
     /* RFU (Reserved for Further Use) */
     RRCTL_RESERVED		= 0x3f, // 0b111111
   };