Export hts_convertStringSystemToUTF8 from the DLL

Mark the CP_ACP->UTF-8 converter HTSEXT_API, matching hts_argv_utf8 next
to it, so WinHTTrack can call the engine's converter instead of keeping a
private copy. Windows-only (inside the _WIN32 block), so no POSIX ABI
change and no soname bump.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

src/htscharset.c

@@ -385,7 +385,7 @@ char *hts_convertStringFromUTF8(const char *s, size_t size, const char *charset)
   return hts_convertStringCPFromUTF8(s, size, cp);
 }
 
-char *hts_convertStringSystemToUTF8(const char *s, size_t size) {
+HTSEXT_API char *hts_convertStringSystemToUTF8(const char *s, size_t size) {
   return hts_convertStringCPToUTF8(s, size, GetACP());
 }
 

src/htscharset.h

@@ -175,7 +175,7 @@ extern char *hts_convertUCS2StringToUTF8(LPWSTR woutput, int wsize);
  * Convert current system codepage to UTF-8.
  * This function is WIN32 specific.
  **/
-extern char *hts_convertStringSystemToUTF8(const char *s, size_t size);
+HTSEXT_API char *hts_convertStringSystemToUTF8(const char *s, size_t size);
 
 /**
  * Replace the CRT's ANSI argv by a UTF-8 one decoded from the real UTF-16