trau: Fix compiler warning about decoded_trau_frame

In file included from bsc_api.c:34:0:
../../include/openbsc/trau_mux.h:62:15: warning: ‘struct decoded_trau_frame’ declared inside parameter list [enabled by default]
  const struct decoded_trau_frame *tf);
               ^
../../include/openbsc/trau_mux.h:62:15: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
../../include/openbsc/trau_mux.h:64:15: warning: ‘struct decoded_trau_frame’ declared inside parameter list [enabled by default]
  const struct decoded_trau_frame *tf);
               ^
../../include/openbsc/trau_mux.h:66:2: warning: ‘struct decoded_trau_frame’ declared inside parameter list [enabled by default]
  const unsigned char *data);
  ^
../../include/openbsc/trau_mux.h:68:2: warning: ‘struct decoded_trau_frame’ declared inside parameter list [enabled by default]
  const unsigned char *data);

openbsc/include/openbsc/trau_mux.h

@@ -32,6 +32,8 @@
 #include <openbsc/gsm_data.h>
 #include <openbsc/mncc.h>
 
+struct decoded_trau_frame;
+
 /* map a TRAU mux map entry */
 int trau_mux_map(const struct gsm_e1_subslot *src,
 		 const struct gsm_e1_subslot *dst);