Fix parallel build in docs/ directory

Fix parallel build in docs/ directory

Every now & then, with parallel builds, we get a failure to
validate hvsupport.html.in.  I eventually noticed that this
is because we get 2 instances of the generator running at
once.

We already list hvsupport.html.in in BUILT_SOURCES but this
was not working. It turns out the flaw is that we were
adding deps to the 'all:' target instead of the 'all-am:'
target. BUILT_SOURCES is a dep of 'all', so any custom
targets written in Makefile.am must use 'all-am:' so that
they don't get run until BUILT_SOURCES are completely
generated

* docs/Makefile.am: s/all/all-am/
(cherry picked from commit 4f4b496e782dd20d105b89f89e07288772555346)