libiconv: disable building the preloadable library

We were already removing the preloadable iconv library (that can be
used through LD_PRELOAD to override the C library iconv
implementation) from staging/target, but it was still built. And this
causes issues in static only scenarios, so this patch changes that to
not even build/install the preloadable library.

[Thomas: changed Gustavo's patch to take into the fact that we never
need the preloadable library.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>