Cast void * to user_fpsimd_state *.

Cast void * to user_fpsimd_state *.

This patch fixes the following build error in GDBserver,

gdb/gdbserver/linux-aarch64-low.c: In function 'void aarch64_fill_fpregset(regcache*, void*)':
gdb/gdbserver/linux-aarch64-low.c:134:38: error: invalid conversion from 'void*' to 'user_fpsimd_state*' [-fpermissive]
   struct user_fpsimd_state *regset = buf;
                                      ^
gdb/gdbserver/linux-aarch64-low.c: In function 'void aarch64_store_fpregset(regcache*, const void*)':
gdb/gdbserver/linux-aarch64-low.c:146:44: error: invalid conversion from 'const void*' to 'const user_fpsimd_state*' [-fpermissive]
   const struct user_fpsimd_state *regset = buf;
                                            ^
gdb/gdbserver:

2015-11-19  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_fill_fpregset): Cast buf to
	struct user_fpsimd_state *.
	(aarch64_store_fpregset): Likewise.