signal: Document glibc's si_code of SI_ASYNCNL

The header uapi/asm-generic/siginfo.h appears to the the repository of
all of these definitions in linux so collect up glibcs additions as
well.  Just to prevent someone from accidentally creating a conflict
in the future.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/uapi/asm-generic/siginfo.h

@@ -165,6 +165,7 @@ typedef struct siginfo {
 #define SI_SIGIO	-5		/* sent by queued SIGIO */
 #define SI_TKILL	-6		/* sent by tkill system call */
 #define SI_DETHREAD	-7		/* sent by execve() killing subsidiary threads */
+#define SI_ASYNCNL	-60		/* sent by glibc async name lookup completion */
 
 #define SI_FROMUSER(siptr)	((siptr)->si_code <= 0)
 #define SI_FROMKERNEL(siptr)	((siptr)->si_code > 0)