drm/gem-shmem: Fix the MODULE_LICENSE() string

drm/gem-shmem: Fix the MODULE_LICENSE() string

Replace the bogus "GPL v2" with "GPL" as MODULE_LICNSE() string. The
value does not declare the module's exact license, but only lets the
module loader test whether the module is Free Software or not.

See commit bf7fbeeae6db ("module: Cure the MODULE_LICENSE "GPL" vs.
"GPL v2" bogosity") in the details of the issue. The fix is to use
"GPL" for all modules under any variant of the GPL.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Fixes: 4b2b5e142ff4 ("drm: Move GEM memory managers into modules")
Link: https://patch.msgid.link/20251209140141.94407-3-tzimmermann@suse.de