storagefile: conditional build of virStorageFileLoadBackendModule

The virStorageFileLoadBackendModule method is only used if either
fs or gluster storage is built in, which doesn't happen on mingw
leading to warning of an unused static function.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/util/virstoragefilebackend.c

@@ -45,7 +45,9 @@ VIR_LOG_INIT("storage.storage_source_backend");
 static virStorageFileBackendPtr virStorageFileBackends[VIR_STORAGE_BACKENDS_MAX];
 static size_t virStorageFileBackendsCount;
 
-#define STORAGE_FILE_MODULE_DIR LIBDIR "/libvirt/storage-file"
+#if WITH_STORAGE_DIR || WITH_STORAGE_FS || WITH_STORAGE_GLUSTER
+
+# define STORAGE_FILE_MODULE_DIR LIBDIR "/libvirt/storage-file"
 
 static int
 virStorageFileLoadBackendModule(const char *name,
@@ -69,7 +71,7 @@ virStorageFileLoadBackendModule(const char *name,
 
     return ret;
 }
-
+#endif /* WITH_STORAGE_DIR || WITH_STORAGE_FS || WITH_STORAGE_GLUSTER */
 
 static int virStorageFileBackendOnceInit(void)
 {