Ever since commit c964b6aa, make was trying to find the timestamp
of '""./apibuild.py".stamp"', but only touching 'apibuild.py.stamp',
and thus always rebuilding.  Reported by Daniel P. Berrange.

* docs/Makefile.am (APIBUILD, APIBUILD_STAMP): Omit bogus quotes.
(cherry picked from commit c0057d9a)