[trace] Minor fixes found during integration

* Mark `ZSTD_CCtx_getParameter()` as const
* Add `extern "C"` guards to `zstd_trace.h`

lib/common/zstd_trace.h

@@ -11,6 +11,10 @@
 #ifndef ZSTD_TRACE_H
 #define ZSTD_TRACE_H
 
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 #include <stddef.h>
 
 /* weak symbol support */
@@ -141,4 +145,8 @@ void ZSTD_trace_decompress_end(
 
 #endif /* ZSTD_TRACE */
 
+#if defined (__cplusplus)
+}
+#endif
+
 #endif /* ZSTD_TRACE_H */