ptrace.h: add __ prefix to ptrace_peeksiginfo_args

ptrace.h: add __ prefix to ptrace_peeksiginfo_args

All the other ptrace structures in this file have a __ prefix except this
new one.  This in turn causes build problems for most packages that try to
use ptrace such as strace:

gcc -DHAVE_CONFIG_H -I. -I../..  -I../../linux/x86_64 -I../../linux \
	-I./linux  -Wall -Wwrite-strings -g -O2 -MT process.o -MD -MP \
	-MF .deps/process.Tpo -c -o process.o ../../process.c
In file included from ../../process.c:63:0:
/usr/include/linux/ptrace.h:58:8: error: redefinition of 'struct ptrace_peeksiginfo_args'
 struct ptrace_peeksiginfo_args {
        ^
In file included from ../../defs.h:159:0,
                 from ../../process.c:37:
/usr/include/sys/ptrace.h:191:8: note: originally defined here
 struct ptrace_peeksiginfo_args
        ^

Since this struct was introduced in glibc-2.18, there shouldn't be any
real regressions with adding the __ prefix.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>