docs: fix generation of docs from VPATH build

docs: fix generation of docs from VPATH build

When generating docs in a VPATH build we get a failure to
create a file due to the 'internals' subdir not existing:

  Generating internals/locking.html.tmp
  /bin/sh: line 3: internals/locking.html.tmp: No such file or directory
  rm: cannot remove ‘internals/locking.html.tmp’: No such file or directory
  Makefile:2229: recipe for target 'internals/locking.html.tmp' failed
  make: *** [internals/locking.html.tmp] Error 1

For some reason, make has decided to run the target

  %.html.tmp: %.html.in site.xsl page.xsl sitemap.html.in $(acl_generated)

instead of the target

  internals/%.html.tmp: internals/%.html.in subsite.xsl page.xsl sitemap.html.in

Removing '$(acl_generated)' from the first target, inexplicably
causes make to now run the correct target for the internals/
files.

Rather than figure this out, lets just combine the two targets
into one.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>