linux-user/main.c: Mark end_exclusive() as possibly unused

The function end_exclusive() isn't used on all targets; mark it as
such to avoid a clang warning.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/main.c

@@ -169,7 +169,7 @@ static inline void start_exclusive(void)
 }
 
 /* Finish an exclusive operation.  */
-static inline void end_exclusive(void)
+static inline void __attribute__((unused)) end_exclusive(void)
 {
     pending_cpus = 0;
     pthread_cond_broadcast(&exclusive_resume);