Cast void * to struct user_pt_regs *

Cast void * to struct user_pt_regs *

This patch fixes the following GDBserver build errors in C++.

gdb/gdbserver/linux-aarch64-low.c:108:33: error: invalid conversion from 'void*' to 'user_pt_regs*' [-fpermissive]
   struct user_pt_regs *regset = buf;
                                 ^
gdb/gdbserver/linux-aarch64-low.c: In function 'void aarch64_store_gregset(regcache*, const void*)':
gdb/gdbserver/linux-aarch64-low.c:121:39: error: invalid conversion from 'const void*' to 'const user_pt_regs*' [-fpermissive]
   const struct user_pt_regs *regset = buf;

gdb/gdbserver:

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

	* linux-aarch64-low.c (aarch64_fill_gregset): Cast buf to
	struct user_pt_regs *.
	(aarch64_store_gregset): Likewise.