Explicitly require gnulib getline module.

Explicitly require gnulib getline module.

Currently, the gnulib getline module is only pulled in indirectly via
the readline module.  As a result, when configuring for a system that
has GNU readline installed (and readline is enabled), the readline
replacement is not used and therefore the getline replacement is
never included.

But on systems that lack getline in the C library, without the gnulib
replacement the build will fail.  Simply listing getline as a needed
module suffices to allow such configurations to build successfully.