subsys/testsuite/coverage: add empty function __gcov_exit()

This is to fix build errors complaining about undefined reference
to __gcov_exit(). There is no special processing required here
so leave the function empty.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

subsys/testsuite/coverage/coverage.c

@@ -42,6 +42,11 @@ void __gcov_merge_add(gcov_type *counters, unsigned int n_counters)
 	/* Unused. */
 }
 
+void __gcov_exit(void)
+{
+	/* Unused. */
+}
+
 /**
  * buff_write_u64 - Store 64 bit data on a buffer and return the size
  */