Use isGV_with_GP in util.c:fbm_compile

Use isGV_with_GP in util.c:fbm_compile

This little statement:

    if (SvSCREAM(sv))
	return;

filters out not only studied strings, but also GVs.

Since studied strings are gone, it only filters out GVs now.  Since
fbm_compile coerces its argument, we still want to filter out GVs, so
we use isGV_with_GP to make the intent clear.