docs: Pass relative paths to apibuild.py

Since commit d195cffa, both $(srcdir) and $(abs_builddir)
are passed to the apibuild.py script; however, since the
former is a relative path and the latter an absolute one, the
script might not be able to detect whether they point to the
same location.

Pass both as relative paths to avoid the issue.

docs/Makefile.am

@@ -325,7 +325,7 @@ $(APIBUILD_STAMP): $(srcdir)/apibuild.py \
 		$(top_srcdir)/src/util/virerror.c \
 		$(top_srcdir)/src/util/virevent.c \
 		$(top_srcdir)/src/util/virtypedparam.c
-	$(AM_V_GEN)srcdir=$(srcdir) builddir=$(abs_builddir) $(PYTHON) $(APIBUILD)
+	$(AM_V_GEN)srcdir=$(srcdir) builddir=$(builddir) $(PYTHON) $(APIBUILD)
 	touch $@