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-xfhSigned-off-by: Dan Williams <dan.j.williams@intel.com>

include/linux/memregion.h

@@ -16,7 +16,7 @@ static inline int memregion_alloc(gfp_t gfp)
 {
 	return -ENOMEM;
 }
-void memregion_free(int id)
+static inline void memregion_free(int id)
 {
 }
 #endif