swconfig: Add missing include

Fixes these warnings:

swlib.c:455:18: warning: implicit declaration of function 'isspace'
swlib.c:461:9: warning: implicit declaration of function 'isdigit'

Signed-off-by: Rosen Penev <rosenp@gmail.com>

package/network/config/swconfig/Makefile

@@ -8,7 +8,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=swconfig
-PKG_RELEASE:=11
+PKG_RELEASE:=12
 
 PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 PKG_LICENSE:=GPL-2.0

package/network/config/swconfig/src/swlib.c

@@ -16,6 +16,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <ctype.h>
 #include <inttypes.h>
 #include <errno.h>
 #include <stdint.h>