memregion: Fix memregion_free() fallback definition

In the CONFIG_MEMREGION=n case, memregion_free() is meant to be a static
inline. 0day reports:

    In file included from drivers/cxl/core/port.c:4:
    include/linux/memregion.h:19:6: warning: no previous prototype for
    function 'memregion_free' [-Wmissing-prototypes]

Mark memregion_free() static.

Fixes: 33dd7075

 ("lib: Uplevel the pmem "region" ida to a global allocator")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://lore.kernel.org/r/165601455171.4042645.3350844271068713515.stgit@dwillia2-xfh
Signed-off-by: Dan Williams <dan.j.williams@intel.com>