Fix fmgroids.h not regenerated after "clean" (not "clean dist") on Windows

Hi all

There's an issue with MSVC builds on Windows where clean.bat deletes
src\include\utils\fmgroids.h (as it should) but build.pl doesn't
re-create it reliably.

It's created fine on the first build because Gen_fmgrtab.pl is called if
src\backend\utils\fmgrtab.c is missing, which it is on first build.
Generating it also generates src\backend\utils\fmgroids.h, which is
copied to src\include. However, this copy isn't repeated after clean
deletes src\include\utils\fmgroids.h .

"clean dist" is fine, since it deletes fmgrtab.c too, causing the whole
thing to be re-generated.

The attached patch fixes the issue.

--
Craig Ringer