Invoke lcov in a way that actually works

It only accepts one directory argument.

(This still doesn't actually work unless you hack ax_code_coverage.m4
to make it allow the version of lcov we ship, but that's one yak too
far right now.)

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4622

Makefile.am

@@ -700,9 +700,7 @@ DISTCLEANFILES += $(pkgconfig_DATA)
 # Code coverage
 @CODE_COVERAGE_RULES@
 CODE_COVERAGE_DIRECTORY = \
-	$(top_builddir)/traprain-service \
-	$(top_builddir)/traprain-client \
-	$(top_builddir)/mock-service \
+	$(top_builddir) \
 	$(NULL)
 CODE_COVERAGE_LCOV_OPTIONS = --base-directory $(abs_top_srcdir)