knxd: fix musl compatibility

Signed-off-by: Othmar Truniger <github@truniger.ch>

net/knxd/patches/0100-musl-compat

@@ -0,0 +1,10 @@
+--- a/src/examples/common.h	2015-06-27 15:20:15.266563893 +0200
++++ b/src/examples/common.h	2015-06-27 15:23:09.406457392 +0200
+@@ -20,6 +20,7 @@
+ #include <stdlib.h>
+ #include <errno.h>
+ #include <unistd.h>
++#include <sys/select.h>
+ #include "eibclient.h"
+ 
+ /** unsigned char*/