fcntl.h: add F_SEAL_FUTURE_WRITE from linux v5.1

needed for android so it can migrate from its ashmem to memfd.
allows making the memfd readonly for future users while keeping
a writable mmap of it. see

  linux commit ab3948f58ff841e51feb845720624665ef5b7ef3
  mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd

include/fcntl.h

@@ -129,6 +129,7 @@ int posix_fallocate(int, off_t, off_t);
 #define F_SEAL_SHRINK	0x0002
 #define F_SEAL_GROW	0x0004
 #define F_SEAL_WRITE	0x0008
+#define F_SEAL_FUTURE_WRITE	0x0010
 
 #define F_GET_RW_HINT		1035
 #define F_SET_RW_HINT		1036