posix: do not unload plugins with GCC ASan

Like valgrind, the address sanitizer generates unusable leak reports if
plugins are unloaded.

src/posix/plugin.c

@@ -78,11 +78,15 @@ int module_Load (vlc_object_t *p_this, const char *path,
  */
 void module_Unload( module_handle_t handle )
 {
+#if !defined(__SANITIZE_ADDRESS__)
 #ifdef HAVE_VALGRIND_VALGRIND_H
     if( RUNNING_ON_VALGRIND > 0 )
         return; /* do not dlclose() so that we get proper stack traces */
 #endif
     dlclose( handle );
+#else
+    (void) handle;
+#endif
 }
 
 /**