nat/linux-ptrace.c: add missing gdb_byte* cast

nat/linux-ptrace.c: add missing gdb_byte* cast

On noMMU platforms, the following code gets compiled:

  child_stack = xmalloc (STACK_SIZE * 4);

Where child_stack is a gdb_byte*, and xmalloc() returns a void*. While
the lack of cast is valid in C, it is not in C++, causing the
following build failure:

../nat/linux-ptrace.c: In function 'int linux_fork_to_function(gdb_byte*, int (*)(void*))':
../nat/linux-ptrace.c:273:29: error: invalid conversion from 'void*' to 'gdb_byte* {aka unsigned char*}' [-fpermissive]
       child_stack = xmalloc (STACK_SIZE * 4);

Therefore, this commit adds the appropriate cast.

gdb/ChangeLog:

	* nat/linux-ptrace.c (linux_fork_to_function): Add cast to
	gdb_byte*.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>