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.

v2: inference rules can't have additional prerequisites so add a target
rule to still depend on gen_pack_header.py

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
1 file changed