add flock64 alias for (struct) flock in fcntl.h

this was a missing part of the LFS64 API; it's "needed" for use with
fcntl and the corresponding lock commands.

include/fcntl.h

@@ -161,6 +161,7 @@ ssize_t tee(int, int, size_t, unsigned);
 #define F_GETLK64 F_GETLK
 #define F_SETLK64 F_SETLK
 #define F_SETLKW64 F_SETLKW
+#define flock64 flock
 #define open64 open
 #define openat64 openat
 #define creat64 creat