x11-libs/pango: fix src_prepare chaining for correct env cleanup

x11-libs/pango: fix src_prepare chaining for correct env cleanup

Properly call up to eclass exported src_prepare functions with the
pango-view.1.in copy necessitated custom src_prepare, so that
xdg_environment_reset gets called.
This fixes build for people who for some reason use `su` instead
of more conceptually correct `su -` (former leaks users env vars
to root session). But this is one of the reasons xdg_environment_reset
exists for correct use instead of my foolish mistakes :)

Closes: https://bugs.gentoo.org/664736
Package-Manager: Portage-2.3.48, Repoman-2.3.10