freerdp-extension-xpunlimited: inherit gitpkgv

If you refer to GITPKGVTAG you need to inherit gitpkgv or the result is
a parsing error.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>