Fix:

error: templates must have C++ linkage

https://github.com/kisli/vmime/issues/218
https://github.com/kisli/vmime/commit/7168f3c4eebcdc0c27d715adbf3d2a22a93d800b
--- src/vmime/charsetConverter_icu.cpp.orig	2023-08-17 18:21:08.000000000 -0500
+++ src/vmime/charsetConverter_icu.cpp	2023-08-17 18:21:38.000000000 -0500
@@ -34,15 +34,12 @@
 #include "vmime/utility/outputStreamStringAdapter.hpp"
 
 
-extern "C"
-{
 #ifndef VMIME_BUILDING_DOC
 
 	#include <unicode/ucnv.h>
 	#include <unicode/ucnv_err.h>
 
 #endif // VMIME_BUILDING_DOC
-}
 
 
 #include <unicode/unistr.h>