decoder_api.h, ...: add "extern C"

src/decoder_api.h

@@ -38,6 +38,10 @@
 
 #include <stdbool.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * Notify the player thread that it has finished initialization and
  * that it has read the song's meta data.
@@ -168,4 +172,8 @@ void
 decoder_mixramp(struct decoder *decoder,
 		char *mixramp_start, char *mixramp_end);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif