graphics/php7*-gd: fix broken build when XPM option is disabled

The switch from bundled gd to graphics/gd broke the port
if a user disabled the XPM option.

PR: 230234 217222
Submitted by: Guido Falsi <madpilot@FreeBSD.org>
Reported by: Guido Falsi <madpilot@FreeBSD.org>, Dani <i.dani@outlook.com>