mtd: export new mtd_mmap_capabilities

The recently added mtd_mmap_capabilities can be used from loadable
modules, in particular romfs, but is not exported, so we get

ERROR: "mtd_mmap_capabilities" [fs/romfs/romfs.ko] undefined!

This adds the missing export.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b4caecd4 ("fs: introduce f_op->mmap_capabilities for nommu mmap support")
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>