mimalloc: avoid %z format

This format is not supported by MSVC runtimes targeted by the MINGW
toolchain.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mimalloc/options.c

@@ -346,7 +346,7 @@ void _mi_fprintf( mi_output_fun* out, void* arg, const char* fmt, ... ) {
 static void mi_vfprintf_thread(mi_output_fun* out, void* arg, const char* prefix, const char* fmt, va_list args) {
   if (prefix != NULL && strlen(prefix) <= 32 && !_mi_is_main_thread()) {
     char tprefix[64];
-    snprintf(tprefix, sizeof(tprefix), "%sthread 0x%zx: ", prefix, _mi_thread_id());
+    snprintf(tprefix, sizeof(tprefix), "%sthread 0x%llx: ", prefix, (unsigned long long)_mi_thread_id());
     mi_vfprintf(out, arg, tprefix, fmt, args);
   }
   else {