oc2g: Add get_hwversion_desc to header file

it's used in oc2g/main.c and it needs to be in a header file.

Similar as previously done for lc15 in 19795c5a.

Change-Id: Ic6826d8c8ff5c648158493454a80704bb956b51d

src/osmo-bts-oc2g/misc/oc2gbts_bid.h

@@ -43,5 +43,6 @@ enum oc2gbts_gsm_band {
 void oc2gbts_rev_get(char *rev_maj, char *rev_min);
 int oc2gbts_model_get(void);
 int oc2gbts_option_get(enum oc2gbts_option_type type);
+const char* get_hwversion_desc();
 
 #endif