gnupg2: Needs a suitable iconv implementation

gnupg2 needs a toolchain with locale support or a package which provides
a suitable iconv implementation (libiconv). Otherwise it will fail at
the configure phase with an error like this one:

*** It is now required to build with support for iconv
*** Please install a suitable iconv implementation.

Fixes:
  http://autobuild.buildroot.net/results/8c9/8c93c28533dfebffa8b2e34b1421d3fa3cdeb278/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Markos Chandras <Markos.Chandras@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>