Makefile.local: Fix doc build by adding missing tomli build dependency

It is a dependency required to build psycopg-c 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

@@ -26,6 +26,9 @@ BUILD_DEPS += configobj
 # swh.docs
 BUILD_DEPS += pifpaf
 
+# psycopg-c
+BUILD_DEPS += tomli
+
 pip-install-swh-dev:
 	python3 -m pip install --upgrade pip wheel setuptools setuptools-scm
 	python3 -m pip install --upgrade $(BUILD_DEPS)