Commit 65d8c92a fixed the mingw spec file for 32-bit builds,
but forgot to make the adjustment for 64-bit builds:

Checking for unpackaged file(s): /usr/lib/rpm/check-files /home/eblake/rpmbuild/BUILDROOT/mingw-libvirt-1.2.7-1.fc20.eblake1404944503.x86_64
error: Installed (but unpackaged) file(s) found:
   /usr/x86_64-w64-mingw32/sys-root/mingw/lib/pkgconfig/libvirt-lxc.pc
   /usr/x86_64-w64-mingw32/sys-root/mingw/lib/pkgconfig/libvirt-qemu.pc

* mingw-libvirt.spec.in (%files): List missing .pc files.
Signed-off-by: NEric Blake <eblake@redhat.com>