gallium/aux/util/u_debug.h: Fix "noreturn" warnings in debug mode

Only decorate function as noreturn when DEBUG is not defined, because
when compiled in DEBUG mode the function actually executes an int3 and
may return, fixes:
u_debug.c: In function '_debug_assert_fail':
u_debug.c:309:1: warning: 'noreturn' function does return

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/gallium/auxiliary/util/u_debug.h

@@ -170,8 +170,8 @@ void _debug_assert_fail(const char *expr,
                         const char *file, 
                         unsigned line, 
                         const char *function)
-#ifdef __GNUC__
-   __attribute__((__noreturn__))
+#if defined(__GNUC__) && !defined(DEBUG)
+   __attribute__((noreturn))
 #endif
 ;