ptrace: check, that PTRACE_LISTEN isn't defined yet

In file included from arch/x86/crtools.c:11:0:
include/ptrace.h:16:0: error: "PTRACE_LISTEN" redefined [-Werror]
 #define PTRACE_LISTEN  0x4208
 ^
In file included from include/ptrace.h:5:0,
                 from arch/x86/crtools.c:11:
/usr/include/sys/ptrace.h:150:0: note: this is the location of the previous definition
 #define PTRACE_LISTEN PTRACE_LISTEN
 ^
cc1: all warnings being treated as errors
make[1]: *** [arch/x86/crtools.o] Error 1
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/ptrace.h

@@ -13,7 +13,9 @@
 # define PTRACE_INTERRUPT	0x4207
 #endif
 
+#ifndef PTRACE_LISTEN
 #define PTRACE_LISTEN		0x4208
+#endif
 
 #ifndef PTRACE_PEEKSIGINFO
 #define PTRACE_PEEKSIGINFO      0x4209