x265: include <sys/stat.h> for FreeBSD

`mode_t` values are defined in <sys/stat.h> in FreeBSD.
They are used for calling open(2) in `source/common/ringmem.cpp`.
The prototype of open(2) is in <fctl.h>,
it's nice to put <sys/stat.h> close to it.

contrib/x265/P00-freebsd-common-threading.patch

@@ -0,0 +1,12 @@
+--- a/source/common/threading.h	2022-02-16 10:18:38.000000000 +0900
++++ b/source/common/threading.h	2022-03-02 11:03:42.002417000 +0900
+@@ -37,6 +37,9 @@
+ #include <semaphore.h>
+ #include <errno.h>
+ #include <fcntl.h>
++#ifdef __FreeBSD__
++#include <sys/stat.h>
++#endif
+ #endif
+
+ #if MACOS