Makefile.local: Fix doc build by adding missing flit-core build dependency

It is a dependency required to build docutils >= 0.21 that does not get
installed in the virtualenv used to build the documentation as we use the
pip option --no-build-isolation that skips installation of build dependencies
specified by PEP 518.

Makefile.local

@@ -29,6 +29,9 @@ BUILD_DEPS += pifpaf
 # psycopg-c
 BUILD_DEPS += tomli
 
+# docutils >= 0.21
+BUILD_DEPS += flit-core
+
 pip-install-swh-dev:
 	python3 -m pip install --upgrade pip wheel setuptools setuptools-scm
 	python3 -m pip install --upgrade $(BUILD_DEPS)