genxml: avoid using a GNU make pattern rule

% pattern rules are a GNU extension.  Convert the use of one to a
inference rule to allow this to build on OpenBSD.

This is a related change to the one made in
e3d43dc5eae5271e2c87bab702aa7409d3dd0b23

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed