exiv2: fix for darwin by providing libiconv

(cherry picked from commit 698d433e74a6dcc5d02e1d1f43083ca97433069b)

pkgs/development/libraries/exiv2/default.nix

@@ -10,6 +10,7 @@
 , doxygen
 , graphviz
 , libxslt
+, libiconv
 }:
 
 stdenv.mkDerivation rec {
@@ -33,6 +34,8 @@ stdenv.mkDerivation rec {
     libxslt
   ];
 
+  buildInputs = lib.optional stdenv.isDarwin libiconv;
+
   propagatedBuildInputs = [
     expat
     zlib