Replace (void *) cast with (gdb_byte *)

Replace (void *) cast with (gdb_byte *)

Fixes:

/home/simark/src/binutils-gdb/gdb/nat/linux-ptrace.c:207:6: error: invalid conversion from ‘void*’ to ‘gdb_byte* {aka unsigned char*}’ [-fpermissive]
   pc = (void *) (uintptr_t) l;
         ^

gdb/ChangeLog:

	* nat/linux-ptrace.c (linux_ptrace_test_ret_to_nx): Replace
	(void *) cast with (gdb_byte *).