devel/appstream-glib: Fix typo

MANPAGE_BUILD_DEPENDS -> MANPAGES_BUILD_DEPENDS

Depending on option selection in dependent ports docbook-{xsl,xml}
might not be installed in the build environment and building with
MANPAGES=on would fail to configure.

PR: 226029
Submitted by: w.schwarzenfeld@utanet.at
Approved by: gnome (maintainer timeout, 2 weeks+)