[libc] Allow strtoul() to interpret negative numbers

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/core/runtime.c

@@ -27,6 +27,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
+#include <ctype.h>
 #include <errno.h>
 #include <assert.h>
 #include <ipxe/init.h>