signal.h: add SYS_USER_DISPATCH si_code value from linux v5.11

see

  linux commit 1d7637d89cfce54a4f4a41c2325288c2f47470e8
  signal: Expose SYS_USER_DISPATCH si_code type

include/signal.h

@@ -263,6 +263,7 @@ void (*sigset(int, void (*)(int)))(int);
 typedef void (*sig_t)(int);
 
 #define SYS_SECCOMP 1
+#define SYS_USER_DISPATCH 2
 #endif
 
 #ifdef _GNU_SOURCE