Include s12z-opc.h using ../opcodes.

Include s12z-opc.h using ../opcodes.

This file exists in binutils-gdb/opcodes, and nothing in Makefile.in adds this
to the include path.  Default builds work because in-tree readline adds
binutils-gdb/ to the include path (!). However, this is broken when compiling
with system readline.

Other files already use this way to include files in opcodes:
./gdb/lm32-tdep.c:#include "../opcodes/lm32-desc.h"
./gdb/or1k-tdep.h:#include "../opcodes/or1k-desc.h"
./gdb/mep-tdep.c:#include "../opcodes/mep-desc.h"

etc.

gdb/ChangeLog:

2019-08-01  Christian Biesinger  <cbiesinger@google.com>

	* s12z-tdep.c: Fix include path for s12z-opc.h.