Fix invalid conversion from void * to gdb_byte *

Fix invalid conversion from void * to gdb_byte *

This patch fixes the following GDB build error in C++ mode.

gdb/nat/linux-ptrace.c: In function 'int linux_child_function(void*)':
gdb/nat/linux-ptrace.c:323:65: error: invalid conversion from 'void*' to 'gdb_byte* {aka unsigned char*}' [-fpermissive]
   linux_fork_to_function (child_stack, linux_grandchild_function);
                                                                 ^

gdb:

2016-01-12  Yao Qi  <yao.qi@linaro.org>

	* nat/linux-ptrace.c (linux_child_function): Cast child_stack
	to gdb_byte * and pass to linux_fork_to_function.