Include header to declare gsm_a_rr_rxlev_vals_ext.

Change-Id: Ie3fd593ef5ce90dc07037f44fda869c26616afd9
Reviewed-on: https://code.wireshark.org/review/18697
Reviewed-by: Guy Harris <guy@alum.mit.edu>

epan/dissectors/packet-gsm_abis_pgsl.c

@@ -32,6 +32,7 @@
 #include <epan/prefs.h>
 
 #include "packet-gsm_rlcmac.h"
+#include "packet-gsm_a_common.h"
 
 void proto_register_abis_pgsl(void);
 void proto_reg_handoff_abis_pgsl(void);