Add missing declaration

Author: bkmgit

modules/opusvoip/opusudp.scm

@@ -63,6 +63,7 @@ void opusvoip_phone_hangup();
   #include <sys/socket.h>
 #else
   #include <sys/fcntl.h>
+  #include <sys/unistd.h>
 #endif
 
 #ifdef IOS