do warning suppression for soundlib vars another way

Instead of introducing a bunch of preprocessor #ifdef blocks,
this approach is less-intrusive.

src/mthrowu.c

@@ -1235,13 +1235,11 @@ hit_bars(
         }
     } else {
         if (!Deaf) {
-#ifdef SND_LIB_INTEGRATED
-            static enum sound_effect_entries se[] = {
+            static enum sound_effect_entries se[] SOUNDLIBONLY = {
                 se_zero_invalid,
                 se_bars_whang, se_bars_whap, se_bars_flapp,
                 se_bars_clink, se_bars_clonk
             };
-#endif
             static const char *const barsounds[] = {
                 "", "Whang", "Whap", "Flapp", "Clink", "Clonk"
             };