sdl12-compat: mac, error_dialog(): avoid warning from old Apple gcc versions:

SDL Commits
1 
From d223c862d80f0a757e5bc28a3a0eaf2f183395c2 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Fri, 11 Jun 2021 07:55:10 +0300
Subject: [PATCH] mac, error_dialog(): avoid warning from old Apple gcc
 versions:

src/SDL12_compat_objc.m: In function 'error_dialog':
src/SDL12_compat_objc.m:61: warning: ISO C90 forbids mixed declarations and code
---
 src/SDL12_compat_objc.m | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/SDL12_compat_objc.m b/src/SDL12_compat_objc.m
index 5c90314..79966d2 100644
--- a/src/SDL12_compat_objc.m
+++ b/src/SDL12_compat_objc.m
@@ -49,6 +49,8 @@ SDL12_PRIVATE void sdl12_compat_macos_init(void)
 
 SDL12_PRIVATE void error_dialog(const char *errorMsg)
 {
+    NSAlert *alert;
+
     if (NSApp == nil) {
         ProcessSerialNumber psn = { 0, kCurrentProcess };
         TransformProcessType(&psn, kProcessTransformToForegroundApplication);
@@ -58,7 +60,7 @@ SDL12_PRIVATE void error_dialog(const char *errorMsg)
     }
 
     [NSApp activateIgnoringOtherApps:YES];
-    NSAlert *alert = [[[NSAlert alloc] init] autorelease];
+    alert = [[[NSAlert alloc] init] autorelease];
     alert.alertStyle = NSAlertStyleCritical;
     alert.messageText = @"Fatal error! Cannot continue!";
     alert.informativeText = [NSString stringWithCString:errorMsg encoding:NSASCIIStringEncoding];