libvici: Wrap header in extern "C" to include it from C++

Fixes #2795.

src/libcharon/plugins/vici/libvici.h

@@ -86,6 +86,10 @@
 
 #include <stdio.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * Opaque vici connection contex.
  */
@@ -465,4 +469,8 @@ void vici_init();
  */
 void vici_deinit();
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /** LIBVICI_H_ @}*/