x11-libs/gtk+: Move adwaita-icon-theme to PDEPEND to fix circular dep

x11-libs/gtk+: Move adwaita-icon-theme to PDEPEND to fix circular dep

adwaita-icon-theme requires gtk+:3 at build time to properly generate the
symbolic PNG icons from SVG files with the gtk-encode-symbolic-svg tool,
which is shipped by gtk+:3. Thus adwaita-icon-theme depend in gtk+:3 can
not be a runtime only RDEPEND, but has to be PDEPEND due to PMS rule:
"Runtime dependencies (RDEPEND). These must be installed and usable before
the results of an ebuild merging are treated as usable."
Should be fine to install adwaita-icon-theme afterwards in the order, to
satisfy default theme icons.

Gentoo-bug: 628076
Package-Manager: Portage-2.3.6, Repoman-2.3.2