Because they use their own compilation rule, they don't use the
dependency tracking logic from Makefile.global.  To make sure that
dependency tracking works anyway for the *_srv.o files, depend on
their *.o siblings as well, which do have proper dependencies.  It's a
hack that might fail someday if there is a *_srv.o without a
corresponding *.o, but it works for now (and those would probably go
into src/backend/port/ anyway).