socat: disable documentation build/installation

The documentation generation process requires a special yold2man
program, for which we don't have a package in Buildroot. Since we
generally don't care much about documentation of packages, just adjust
the package Makefile.in to not build/install its documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>