Link digidocpp against iconv to unbreak src/digidoc-tool and tests/TSLTests
build:

	ld: error: undefined symbol: libiconv_open

https://github.com/open-eid/libdigidocpp/pull/470

Index: src/CMakeLists.txt
--- src/CMakeLists.txt.orig
+++ src/CMakeLists.txt
@@ -227,7 +227,7 @@ set_target_properties(digidocpp PROPERTIES
 
 target_link_libraries(digidocpp
     PRIVATE ${CMAKE_DL_LIBS} minizip
-    PUBLIC digidocpp_priv
+    PUBLIC digidocpp_priv Iconv::Iconv
 )
 
 if( BUILD_TOOLS )
