clamav: add libiconv dependencies when build with NLS

NLS means Native Language Support and when you have it enabled (it is
not default), clamav can not be compiled as it shows following error:

Package clamav is missing dependencies for the following libraries:
libiconv.so.2

Also, it is required that package libiconv-full is compiled first/before
than clamav and then try to compile clamav.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>