Spell the autorelease pool in SDL_sysurl.m via SDL_OBJC_POOL.

Same dual-mode treatment as the other src/SDL_objc_compat.h users in
patch 0005: GCC cannot parse @autoreleasepool; the macro keeps the
upstream spelling under clang and drains an NSAutoreleasePool via the
cleanup attribute under GCC - the error-path return included, which a
manual [pool release] conversion has to handle by hand.

--- src/misc/macos/SDL_sysurl.m
+++ src/misc/macos/SDL_sysurl.m
@@ -26,9 +26,11 @@
 
 #import <Cocoa/Cocoa.h>
 
+#include "SDL_objc_compat.h"
+
 bool SDL_SYS_OpenURL(const char *url)
 {
-    @autoreleasepool {
+    SDL_OBJC_POOL {
         CFURLRef cfurl = CFURLCreateWithBytes(NULL, (const UInt8 *)url, SDL_strlen(url), kCFStringEncodingUTF8, NULL);
         OSStatus status = LSOpenCFURLRef(cfurl, NULL);
         CFRelease(cfurl);