net.h: do not include sys/select.h in Windows builds

Windows apparently has no select.h header, therefore it is not
included for Windows builds.

include/coap3/net.h

@@ -14,8 +14,8 @@
 
 #include <stdlib.h>
 #include <string.h>
-#include <sys/select.h>
 #ifndef _WIN32
+#include <sys/select.h>
 #include <sys/time.h>
 #endif
 #include <time.h>