Import FireRef before using it

Author: theotherjimmy

tools/build_api.py

@@ -42,7 +42,7 @@
                     MBED_CONFIG_FILE, MBED_LIBRARIES_DRIVERS,
                     MBED_LIBRARIES_PLATFORM, MBED_LIBRARIES_HAL,
                     BUILD_DIR)
-from .resources import Resources, FileType
+from .resources import Resources, FileType, FileRef
 from .notifier.mock import MockNotifier
 from .targets import TARGET_NAMES, TARGET_MAP
 from .libraries import Library