softmagic: include limits.h for UINT_MAX usage

Platforms like Solaris need this to compile.

Signed-off-by: Fabian Groffen <grobian@gentoo.org>

--- a/src/softmagic.c
+++ b/src/softmagic.c
@@ -42,6 +42,7 @@
 #include <ctype.h>
 #include <stdlib.h>
 #include <time.h>
+#include <limits.h>
 #include "der.h"
 
 file_private int match(struct magic_set *, struct magic *, file_regex_t **, size_t,