uio.h: add RWF_NOAPPEND flag for pwritev2

added in linux kernel commit 73fa7547c70b32cc69685f79be31135797734eb6.

this is added now as a prerequisite for fixing pwrite/pwritev behavior
for O_APPEND files.

include/sys/uio.h

@@ -46,6 +46,7 @@ ssize_t pwritev2 (int, const struct iovec *, int, off_t, int);
 #define RWF_SYNC 0x00000004
 #define RWF_NOWAIT 0x00000008
 #define RWF_APPEND 0x00000010
+#define RWF_NOAPPEND 0x00000020
 #endif
 
 #ifdef __cplusplus