fix gdbserver build in nat/linux-ptrace.c on arm-android

fix gdbserver build in nat/linux-ptrace.c on arm-android

The following change replaced an include of gregset.h by
an include of <sys/procfs.h>:

    commit 39b22471578843019026c50fcdbe0483a6045970
    Date:   Thu Aug 11 12:01:22 2016 +0100
    Subject: Fix fallout from gdb/20413's fix
             (x32: linux_ptrace_test_ret_to_nx: Cannot PTRACE_PEEKUSER)

Unfortunately, this broke gdbserver on Android, because that file
does not exist on this platform.  This patch fixes the issue by
conditionalizing its include with HAVE_SYS_PROCFS_H (which we check
both in gdb/configure and gdbserver/configure).

gdb/ChangeLog:

        * nat/linux-ptrace.c: Only include <sys/procfs.h> if
        HAVE_SYS_PROCFS_H is defined.

Tested by rebuilding gdbserver on arm-android and GNU/Linux.