gem_gtt_hog: use a normal exit()

Since we don't execute any subtests, using igt_exit leads to
inconsistent behaviour. In the future, this may be converted.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73641
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

tests/gem_gtt_hog.c

@@ -141,7 +141,7 @@ static void run(int child)
 	munmap(ptr, size);
 
 	igt_assert(x == canary);
-	igt_exit();
+	exit(0);
 }
 
 int main(int argc, char **argv)