Define enum out of struct

Define enum out of struct

This patch moves the definition of enum out of the scope of struct
aarch64_memory_operand, otherwise it breaks GDB build in c++ mode.

gdb:

2015-10-14  Yao Qi  <yao.qi@linaro.org>

	* arch/aarch64-insn.h (struct aarch64_memory_operand): Move enum
	out of it.
	(enum aarch64_memory_operand_type): New.