x11/gdm: Correct CPE (do not set CPE_VENDOR twice)

There are no CPE that match
cpe:2.3:a:gnome_display_manager:gdm:*:*:*:*:*:*

Correct it to
cpe:2.3:a:gnome:gnome_display_manager:*:*:*:*:*:*:*:*

PR: 234851
Approved by: gnome (maintainer timeout, 2 weeks)
Differential Revision: https://reviews.freebsd.org/D18801