Add OPTIONS_H_EXTRA to GTFILES

Add OPTIONS_H_EXTRA to GTFILES

I have a patch that adds a typedef to aarch64's <cpu>-opts.h.
The typedef is used for a TargetVariable in the .opt file,
which means that it is covered by PCH and so needs to be
visible to gengtype.

<cpu>-opts.h is not included directly in tm.h, but indirectly
by target headers (in this case aarch64.h).  There was therefore
nothing that caused it to be added to GTFILES.

gcc/
	* Makefile.in (GTFILES): Add OPTIONS_H_EXTRA.