[S390] Avoid compile warning.

arch/s390/mm/fault.c: In function `signal_return':
arch/s390/mm/fault.c:256: warning: unused variable `compat'
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/mm/fault.c

@@ -253,7 +253,10 @@ static int signal_return(struct mm_struct *mm, struct pt_regs *regs,
 			 unsigned long address, unsigned long error_code)
 {
 	u16 instruction;
-	int rc, compat;
+	int rc;
+#ifdef CONFIG_COMPAT
+	int compat;
+#endif
 
 	pagefault_disable();
 	rc = __get_user(instruction, (u16 __user *) regs->psw.addr);