Include sys/ptrace.h as early as possible.

Include sys/ptrace.h as early as possible.

On some systems, at least on Fedora 27 ppc64le with glibc 2.26-24 and
kernel 4.14.18-300, including sys/ptrace.h late (after signal.h or
sys/wait.h for example) will cause issues and produce errors like:

In file included from /usr/include/asm/sigcontext.h:12:0,
                 from /usr/include/bits/sigcontext.h:30,
                 from /usr/include/signal.h:287,
                 from /usr/include/sys/wait.h:36,
                 from linux-pid-attach.c:38:
/usr/include/sys/ptrace.h:73:3: error: expected identifier before numeric constant
   PTRACE_GETREGS = 12,
   ^

Swapping the include order fixes these issues.

Signed-off-by: Mark Wielaard <mark@klomp.org>