Fix warning about unused mutex

Move the metrics mutex into the ifdef MBED_STACK_STATS_ENABLED since
it is not used ouside of it. This fixes the warning:
[Warning] greentea_metrics.cpp@37,28: 'mutex' defined but not used [-Wunused-variable]

Author: c1728p9

features/frameworks/greentea-client/source/greentea_metrics.cpp

@@ -32,9 +32,9 @@ typedef struct {
     uint32_t max_stack;
 } thread_info_t;
 
+#if defined(MBED_STACK_STATS_ENABLED) && MBED_STACK_STATS_ENABLED
 // Mutex to protect "buf"
 static SingletonPtr<Mutex> mutex;
-#if defined(MBED_STACK_STATS_ENABLED) && MBED_STACK_STATS_ENABLED
 static char buf[128];
 static SingletonPtr<CircularBuffer<thread_info_t, THREAD_BUF_COUNT> > queue;
 #endif