linux/parser.h: add include guards

<linux/parser.h> is missing include guards.  Add them.

This is needed to allow declaring a function in <linux/fscrypt.h> that
takes a substring_t parameter.

Link: https://lore.kernel.org/r/20200512233251.118314-2-ebiggers@kernel.org
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>