mail: Fix mismatch in array bounds for isign().

Reviewed by:	imp, emaste
Reported by:	GCC -Warray-parameter
Differential Revision:	https://reviews.freebsd.org/D36761

(cherry picked from commit 157436dcbf)

usr.bin/mail/extern.h

@@ -153,7 +153,7 @@ int	 isdate(char *);
 int	 isdir(char *);
 int	 isfileaddr(char *);
 int	 ishead(char *);
-int	 isign(const char *, struct ignoretab []);
+int	 isign(const char *, struct ignoretab [2]);
 int	 isprefix(const char *, const char *);
 void	 istrncpy(char *, const char *, size_t);
 const struct cmd *