boost: fix missing select on BR2_PACKAGE_LIBICONV

The boost package adds a dependency on the libiconv package when
boost-locale is selected but locale support is not available in the
toolchain. However, it forgets to select BR2_PACKAGE_LIBICONV in this
situation, causing the following build failure:

  http://autobuild.buildroot.org/results/ab1809f4fa8cf29208f9503edd90aca87fe397c2/



This commit fixes this mistake by adding the necessary select
BR2_PACKAGE_LIBICONV.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>