Fix crash when trying to load DCP (#7039)

Stream a C-string of the file name instead of the Glib::ustring. The
ustring can throw an exception with certain locale settings and file
name character combinations, such as with LANG=es_ES and the character
"ä".
Only print the message if verbose is true.

rtengine/dcp.cc

@@ -1177,7 +1177,9 @@ DCPProfile::DCPProfile(const Glib::ustring& filename) :
 
     // Color Matrix (one is always there)
     if (!md.find(TAG_KEY_COLOR_MATRIX_1)) {
-        std::cerr << "DCP '" << filename << "' is missing 'ColorMatrix1'. Skipped." << std::endl;
+        if (settings->verbose) {
+            std::cerr << "DCP '" << filename.c_str() << "' is missing 'ColorMatrix1'. Skipped." << std::endl;
+        }
         return;
     }