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>

include/linux/parser.h

@@ -7,7 +7,8 @@
  * but could potentially be used anywhere else that simple option=arg
  * parsing is required.
  */
-
+#ifndef _LINUX_PARSER_H
+#define _LINUX_PARSER_H
 
 /* associates an integer enumerator with a pattern string. */
 struct match_token {
@@ -34,3 +35,5 @@ int match_hex(substring_t *, int *result);
 bool match_wildcard(const char *pattern, const char *str);
 size_t match_strlcpy(char *, const substring_t *, size_t);
 char *match_strdup(const substring_t *);
+
+#endif /* _LINUX_PARSER_H */