libvlc.h: ensure the header is correctly exposed to C++

The header is not public but can be used by tests in C++.

src/libvlc.h

@@ -26,6 +26,10 @@
 
 #include <vlc_input_item.h>
 
+# ifdef __cplusplus
+extern "C" {
+# endif
+
 extern const char psz_vlc_changeset[];
 
 typedef struct variable_t variable_t;
@@ -210,4 +214,8 @@ int vlc_MetadataRequest(libvlc_int_t *libvlc, input_item_t *item,
  */
 void var_OptionParse (vlc_object_t *, const char *, bool trusted);
 
+# ifdef __cplusplus
+} // extern "C"
+# endif
+
 #endif