Include asm/ptrace.h for linux-aarch64-low.c

A recent change to glibc removed asm/ptrace.h from user.h for AArch64.
This meant that cross-native builds of gdbserver using trunk glibc broke
because linux-aarch64-low.c because user_hwdebug_state couldn't be found.

This is like commit #036cd38182bde32d8297b630cd5c861d53b8949e

2014-05-23  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

        * linux-aarch64-low.c (asm/ptrace.h): Include.