sys/ptrace.h: add missing PTRACE_EVENT_STOP

new in linux v3.1 commit 3544d72a0e10d0aa1c1bd59ed77a53a59cdc12f7
changed in linux v3.4 commit 5cdf389aee90109e2e3d88085dea4dd5508a3be7

A tracer recieves this event in the waitpid status of a PTRACED_SEIZED
process.

include/sys/ptrace.h

@@ -84,6 +84,7 @@ extern "C" {
 #define PTRACE_EVENT_VFORK_DONE 5
 #define PTRACE_EVENT_EXIT 6
 #define PTRACE_EVENT_SECCOMP 7
+#define PTRACE_EVENT_STOP 128
 
 #define PTRACE_PEEKSIGINFO_SHARED 1