Remove 'gcc/omp-offload.c' from 'GTFILES'

Given that it doesn't contain any 'GTY' markers, no 'gcc/gt-omp-offload.h' file
gets generated (and '#include'd anywhere).

Small fix-up for r243673 (Git commit 629b3d75)
"Split omp-low into multiple files".

	gcc/
	* Makefile.in (GTFILES): Remove '$(srcdir)/omp-offload.c'.