Makefile: Fix dependency broken in 9a93b49.
Makefile: Fix dependency broken in 9a93b49.

That change added extra dependencies that broke incremental
rebuilds -- use $< to note that the .o files should only directly
use the .c file when rebuilding, but be invalidated by changes to
greatest.h or the Makefile.

silentbicycle/greatest,silentbicycle/greatest