Include insn-opinit.h in PLUGIN_H [PR110610]

This patch fixes PR110610 by including insn-opinit.h in the INTERNAL_FN_H list,
as insn-opinit.h is now required by internal-fn.h. This will lead to
insn-opinit.h being installed in the plugin directory.

gcc/ChangeLog:

	PR plugins/110610
	* Makefile.in (INTERNAL_FN_H): Add insn-opinit.h.