ptrace.h: move PTRACE_{GET,SET}SIGMASK under ifdef

In glibc 2.23 PTRACE_{GET,SET}SIGMASK are defined at <ptrace.h>:
http://fossies.org/diffs/glibc/2.22_vs_2.23/sysdeps/unix/sysv/linux/powerpc/sys/ptrace.h-diff.html

Move them under ifdef to fix the following errors:
In file included from arch/x86/crtools.c:14:0:
~/tools/criu/criu/include/ptrace.h:47:0: error: "PTRACE_GETSIGMASK" redefined [-Werror]
 #define PTRACE_GETSIGMASK 0x420a
 ^
In file included from include/ptrace.h:5:0,
                 from arch/x86/crtools.c:14:
/usr/include/sys/ptrace.h:156:0: note: this is the location of the previous definition
 #define PTRACE_GETSIGMASK PTRACE_GETSIGMASK
 ^
In file included from arch/x86/crtools.c:14:0:
~/tools/criu/criu/include/ptrace.h:48:0: error: "PTRACE_SETSIGMASK" redefined [-Werror]
 #define PTRACE_SETSIGMASK 0x420b
 ^
In file included from include/ptrace.h:5:0,
                 from arch/x86/crtools.c:14:
/usr/include/sys/ptrace.h:159:0: note: this is the location of the previous definition
 #define PTRACE_SETSIGMASK PTRACE_SETSIGMASK
 ^
cc1: all warnings being treated as errors

Fixes #119
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/ptrace.h

@@ -44,8 +44,10 @@ struct ptrace_peeksiginfo_args {
 # define PTRACE_SETREGSET	0x4205
 #endif
 
-#define PTRACE_GETSIGMASK	0x420a
-#define PTRACE_SETSIGMASK	0x420b
+#ifndef PTRACE_GETSIGMASK
+# define PTRACE_GETSIGMASK	0x420a
+# define PTRACE_SETSIGMASK	0x420b
+#endif
 
 #ifndef PTRACE_SECCOMP_GET_FILTER
 #define PTRACE_SECCOMP_GET_FILTER	0x420c