x11-libs/gdk-pixbuf: fix DEPEND
The bump was converted to EAPI-7 initially, so the second occurrence
was a BDEPEND, but then it was realized that gnome-utils.eclass is
broken for EAPI-7 gnome2_gdk_pixbuf_update calls and reverted back
to EAPI-6 with a forgotten ${DEPEND} re-introduction when renaming
BDEPEND back to DEPEND.
No revbump, as due to the ordering, this meant that only build time
deps were wrong, not runtime.
Closes: https://bugs.gentoo.org/702950
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Mart Raudsepp <leio@gentoo.org>