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);
1 file changed
tree: 68d44dde25bf518ae13f2bf8d76c013af21ad1ae
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README