add syscall wrapper for flock

it should be noted that flock does not mix well with standard fcntl
locking, but nonetheless some applications will attempt to use flock
instead of fcntl if both exist. options to configure or small patches
may be needed. debian maintainers have plenty of experience with this
unfortunate situation...

src/linux/flock.c

+#include <sys/file.h>
+#include "syscall.h"
+
+int flock(int fd, int op)
+{
+	return syscall(SYS_flock, fd, op);
+}