Don't use ctype.h routines.

That avoids locale dependency and handles possibly-signed chars (which
we weren't doing before).

Change-Id: I189222eff624ad2d2e960bc0b69f3f22d35f351f
Reviewed-on: https://code.wireshark.org/review/4787
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wsutil/str_util.c

@@ -25,8 +25,6 @@
 #include <glib.h>
 #include "str_util.h"
 
-#include <ctype.h>
-
 int
 ws_xton(char ch)
 {
@@ -103,7 +101,7 @@ isdigit_string(guchar *str)
 
 	/* Loop until we reach the end of the string (a null) */
 	for(pos = 0; str[pos] != '\0'; pos++){
-		if(!isdigit(str[pos])){
+		if(!g_ascii_isdigit(str[pos])){
 			/* The string contains a non-digit character */
 			return FALSE;
 		}