package: swconfig: Fix build failure

Signed-off-by: Phil Sutter <phil@nwl.cc>

package/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>