pkg-perl: fix for Perl XS packages configured by Makefile.PL

ExtUtils::MakeMaker adds all the header files used by the perl as
dependencies to the generated Makefile. This means that the generated
Makefile will depend on the system's header files.

Usually this is not a problem, because when building the target package,
these header files will indeed be found in $(STAGING_DIR). However, some
distro's add an extra header file to the system's perl. This header is
also included in the generated Makefile, which makes the build fail
because it doesn't exist in $(STAGING_DIR).

As a work-around, explicitly create this header file in $(STAGING_DIR).
It doesn't hurt to create it even if the system perl doesn't need it.