Motivation for this change

Changelog:

1. Move libiconv from nativeBuildInputs to buildInputs, fixes:

./libxml/encoding.h:29:10: fatal error: 'iconv.h' file not found
#include <iconv.h>
         ^~~~~~~~~
1 error generated.

2. Add ncurses to nativeBuildInputs, fixes:

Checking for package ncurses using ncurses-config... not found

You need package ncurses in order to compile this program.
Please make sure it is installed.
./config.sh: line 87: curl-config: command not found
./config.sh: line 91: curl-config: command not found

3. Add gettext to buildInputs, fixes:

In file included from src/configcontainer.cpp:10:
./config.h:15:10: fatal error: 'libintl.h' file not found
#include <libintl.h>
         ^~~~~~~~~~~
1 error generated.

4. Fix prefix, otherwise newsboat searches for translations in /usr/local/share

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @dywedir @doronbehar