stdinc: Silence clang warning for -Wimplicit-fallthrough.

In a more ideal world, we'd use the appropriate `__attribute__` here, but
it's one thing in a public header that probably shouldn't be there at all, so
this is good enough for now.

Fixes #4307.

include/SDL_stdinc.h

@@ -481,16 +481,28 @@ SDL_FORCE_INLINE void SDL_memset4(void *dst, Uint32 val, size_t dwords)
     size_t _n = (dwords + 3) / 4;
     Uint32 *_p = SDL_static_cast(Uint32 *, dst);
     Uint32 _val = (val);
-    if (dwords == 0)
+    if (dwords == 0) {
         return;
-    switch (dwords % 4)
-    {
+    }
+
+    /* !!! FIXME: there are better ways to do this, but this is just to clean this up for now. */
+    #ifdef __clang__
+    #pragma clang diagnostic push
+    #pragma clang diagnostic ignored "-Wimplicit-fallthrough"
+    #endif
+
+    switch (dwords % 4) {
         case 0: do {    *_p++ = _val;   /* fallthrough */
         case 3:         *_p++ = _val;   /* fallthrough */
         case 2:         *_p++ = _val;   /* fallthrough */
         case 1:         *_p++ = _val;   /* fallthrough */
         } while ( --_n );
     }
+
+    #ifdef __clang__
+    #pragma clang diagnostic pop
+    #endif
+
 #endif
 }