build: fix VPATH build break from previous patch

Partial revert of commit c3c30d4d.

* docs/Makefile.am (internals/%.html.tmp): Restore MKDIR_P; it is
needed for intermediate file after all.
Reported by Daniel P. Berrange.

docs/Makefile.am

@@ -127,6 +127,7 @@ hvsupport.html.in: $(srcdir)/hvsupport.pl $(srcdir)/../src/libvirt_public.syms \
 internals/%.html.tmp: internals/%.html.in subsite.xsl page.xsl sitemap.html.in
 	@if [ -x $(XSLTPROC) ] ; then \
 	  echo "Generating $@"; \
+	  $(MKDIR_P) internals; \
 	  name=`echo $@ | sed -e 's/.tmp//'`; \
 	  $(XSLTPROC) --stringparam pagename $$name --nonet --html \
 	    $(top_srcdir)/docs/subsite.xsl $< > $@ \