do not include linux/ptrace.h in vgdb.c

Include of linux/ptrace.h was added in revision r11740,
to avoid compilation error on s390x (fedora and suse).

The compilation error was retrieved thanks to archeological research
done by Christian Borntraeger: without this include, the following was given:
  error: 'PT_ENDREGS' undeclared
There was also some errors on ppc64 around the same time:
  error: 'PTRACE_GETREGS' undeclared

Currently, the inclusion of linux/ptrace.h gives a problem on amd64/fedora20:
/usr/include/linux/ptrace.h:58:8: error: redefinition of ‘struct ptrace_peeksiginfo_args’
/usr/include/sys/ptrace.h:191:8: note: originally defined here

According to man ptrace, it is good enough to include sys/ptrace.h
(which should avoid the problem on amd64/f20).

The linux/ptrace.h is deemed not necessary anymore as:
1. Christian has tested on sles11sp2 on s390x.
2. since linux/ptrace.h was added in vgdb.c, #ifdef PT_ENDREGS and
  #ifdef PTRACE_GETREGS were added

=> remove the linux/ptrace.h
(tested on x86/f12, ppc64/f18, amd64/deb6, sles11sp2/s390x)

Thanks to Christian for the investigations



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13471 a5019735-40e9-0310-863c-91ae7b9d1cf9