Fix missing include for flock(2)

On OpenBSD it's in <fcntl.h>, and it gets declared anyway on FreeBSD
and macOS. Curiously, on GNU/Linux, LOCK_* are defined, but flock(2)
isn't declared.

ui.c

@@ -41,6 +41,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sysexits.h>
+#include <sys/file.h>
 #include <term.h>
 #include <termios.h>
 #include <time.h>