Include asm/ptrace.h in aarch64-linux-nat.c

Include asm/ptrace.h in aarch64-linux-nat.c

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

Fixed by including asm/ptrace.h like other ports.

2014-05-22  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

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