pkgconfig: Fix missing libxml2 in Requires.private
pkgconfig: Fix missing libxml2 in Requires.private

As of v0.5, libstoken requires libxml2.  This needs to be explicitly
specified in stoken.pc, similar to nettle/hogweed/tomcrypt.

Signed-off-by: Kevin Cernekee <3105dbb3dd63fbb4f6371cfb04adc6726f76c56b@gmail.com>

cernekee/stoken,mtmiller/stoken,cernekee/stoken,mtmiller/stoken,cernekee/stoken,mtmiller/stoken,mtmiller/stoken,cernekee/stoken