iwcap: add missing include

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r41956

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@41978 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/network/utils/iwcap/src/iwcap.c

@@ -26,6 +26,7 @@
 #include <signal.h>
 #include <syslog.h>
 #include <errno.h>
+#include <byteswap.h>
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/ioctl.h>