fix MSVC build error.

src/stdlib/SDL_string.c

@@ -561,7 +561,7 @@ SDL_utf8strlcpy(SDL_OUT_Z_CAP(dst_bytes) char *dst, const char *src, size_t dst_
     size_t src_bytes = SDL_strlen(src);
     size_t bytes = SDL_min(src_bytes, dst_bytes - 1);
     size_t i = 0;
-    unsigned char trailing_bytes = 0;
+    size_t trailing_bytes = 0;
 
     if (bytes) {
         unsigned char c = (unsigned char)src[bytes - 1];