build: fix build with --enable-doc (docs/api/Makefile.am)

build: fix build with --enable-doc (docs/api/Makefile.am)

"./autogen.sh --enable-doc && make" produced this error:
warning: failed to load external entity "../settings-spec.xml"
../network-manager-docs.xml:57: element include: XInclude error : could not load ../settings-spec.xml, and no fallback was found

Removing settings-spec.xml from $(content_files) made the file non-DISTed but it
also removed the file as a dependency for html-build.stamp that also runs
cd html && gtkdoc-mkhtml $$mkhtml_options $(MKHTML_OPTIONS) $(DOC_MODULE) ../$(DOC_MAIN_SGML_FILE)
and $(DOC_MAIN_SGML_FILE) includes settings-spec.xml.

Fix that by making $(DOC_MAIN_SGML_FILE) dependent on setting-spec.xml.