Building when configured --with-libvirtd=no fails with:

In file included from ../src/qemu/qemu_command.h:30:0,
                 from testutilsqemu.h:4,
                 from networkxml2xmltest.c:14:
../src/qemu/qemu_conf.h:175:5: error: expected specifier-qualifier-list before 'virStateInhibitCallback'

* src/libvirt_internal.h (virStateInhibitCallback): Move outside
of conditional.
Signed-off-by: NEric Blake <eblake@redhat.com>