[vte] build: Silence a warning with debug disabled

[Egmont Koblinger <egmontkob src gnome org>]

commit b588b33b568f7687967e2e512aa65f8375f25163
Author: Egmont Koblinger <egmont gmail com>
Date:   Wed Sep 18 15:34:29 2019 +0200

    build: Silence a warning with debug disabled
    
    https://gitlab.gnome.org/GNOME/vte/issues/173

 src/vte.cc | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/vte.cc b/src/vte.cc
index 1734d34f..79a59ea6 100644
--- a/src/vte.cc
+++ b/src/vte.cc
@@ -3847,12 +3847,14 @@ Terminal::process_incoming()
                         case vte::base::UTF8Decoder::ACCEPT: {
                                 auto rv = m_parser.feed(m_utf8_decoder.codepoint());
                                 if (G_UNLIKELY(rv < 0)) {
+#ifdef DEBUG
                                         uint32_t c = m_utf8_decoder.codepoint();
                                         char c_buf[7];
                                         g_snprintf(c_buf, sizeof(c_buf), "%lc", c);
                                         char const* wp_str = g_unichar_isprint(c) ? c_buf : 
_vte_debug_sequence_to_string(c_buf, -1);
                                         _vte_debug_print(VTE_DEBUG_PARSER, "Parser error on U+%04X [%s]!\n",
                                                          c, wp_str);
+#endif
                                         break;
                                 }