COMMON: Make use of ICONV_CONST

Is #define'd "const" if the iconv() implementation is of type
size_t iconv(iconv_t, const char **, size_t, char **, size_t)
instead of
size_t iconv(iconv_t, char **, size_t, char **, size_t).

This is apparently the case for FreeBSD and PPC-based Mac OS X.

src/common/ustring.cpp

@@ -137,7 +137,7 @@ class ConversionManager : public Singleton<ConversionManager> {
 		iconv(_fromLatin9, 0, 0, 0, 0);
 
 		// Convert
-		if (iconv(_fromLatin9, (char **) &data, &inBytes, (char **) &outBuf, &outBytes) == ((size_t) -1))
+		if (iconv(_fromLatin9, (ICONV_CONST char **) &data, &inBytes, (char **) &outBuf, &outBytes) == ((size_t) -1))
 			warning("Failed completely converting a latin9 string");
 
 		// And this should be our converted string