glconfig.mk: Remove some redundant prerequisites.

glconfig.mk: Remove some redundant prerequisites.

Since the glsym rules are only invoked recursively by a rule that
already has all required dependencies, it is not necessary to have
prerequisites on the individual glsym files to the gnulib headers.

As it turns out these redundant prerequisites trip up "make" on
ULTRIX 4.5 due to a bug expanding $(gnulib_symfiles) (which I do
not fully understand), dropping it basically makes things work
just fine, so no real reason to hang onto it.