Add iconv to EXTRA_LIBS in CMakeLists.

[codeman38]

Compilation via cmake fails on OS X if libiconv is not specified as a required library:

Undefined symbols for architecture x86_64:
  "_iconv", referenced from:
      _EPG_DVB_decode_string in libccx.a(ts_tables_epg.c.o)
  "_iconv_close", referenced from:
      _EPG_DVB_decode_string in libccx.a(ts_tables_epg.c.o)
  "_iconv_open", referenced from:
      _EPG_DVB_decode_string in libccx.a(ts_tables_epg.c.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Adding it to EXTRA_LIBS in CMakeLists.txt is enough to make compilation successful.