fix some fall-through warnings

Author: John Regan

src/stb_image.h

@@ -5240,10 +5240,12 @@ static int stbi__tga_get_comp(int bits_per_pixel, int is_grey, int* is_rgb16)
    switch(bits_per_pixel) {
       case 8:  return STBI_grey;
       case 16: if(is_grey) return STBI_grey_alpha;
-            // else: fall-through
+      /* fall through */
       case 15: if(is_rgb16) *is_rgb16 = 1;
             return STBI_rgb;
-      case 24: // fall-through
+      /* fall through */
+      case 24:
+      /* fall through */
       case 32: return bits_per_pixel/8;
       default: return 0;
    }