Fix reimport of contrib/tools/flex

ref:f81f5b9fb61ea77b538b319aff9754b8968f13c0

build/conf/license.conf

@@ -112,6 +112,7 @@ LICENSES_UNATTRIBUTED= \
     LicenseRef-scancode-cc-by-2.0-uk \
     LicenseRef-scancode-cc-devnations-2.0 \
     LicenseRef-scancode-cc-sampling-1.0 \
+    LicenseRef-scancode-flex-2.5 \
     LicenseRef-scancode-free-unknown \
     LicenseRef-scancode-generic-cla \
     LicenseRef-scancode-generic-exception \