libshout-idjc Warning fix

[daschuer]

I have noticed that we have enabled -Wall for windows which is literal all.
Now there are some real issue warned, but I consider fixing them to risky in a stable build and also considering that we update libshout-idjc one day. See #4723

[uklotzde]

LGTM

[uklotzde]

cc1: warning: command-line option ‘-Woverloaded-virtual’ is valid for C++/ObjC++ but not for C
[28/795] Building C object lib/libshout-idjc/CMakeFiles/shout_mixxx.dir/src/shout.c.o
cc1: warning: command-line option ‘-Woverloaded-virtual’ is valid for C++/ObjC++ but not for C
/home/uk/volumes/Build/cpp/mixxx/lib/libshout-idjc/src/shout.c: In function ‘try_connect’:
/home/uk/volumes/Build/cpp/mixxx/lib/libshout-idjc/src/shout.c:1310:25: warning: this statement may fall through [-Wimplicit-fallthrough=]
 1310 |             self->state = SHOUT_STATE_REQ_CREATION;
      |             ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/home/uk/volumes/Build/cpp/mixxx/lib/libshout-idjc/src/shout.c:1312:9: note: here
 1312 |         case SHOUT_STATE_REQ_CREATION:
      |         ^~~~