util/unicode: allow calling from C++

The omission of the 'extern "C"' machinery seems like an oversight.

util/unicode-util.h

@@ -4,9 +4,16 @@
 #include <stdbool.h>
 #include <gmodule.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* The utf8 encoded string would tokenize as a single word, according
  * to xapian. */
 bool unicode_word_utf8 (const char *str);
 typedef gunichar notmuch_unichar;
 
+#ifdef __cplusplus
+}
+#endif
 #endif