Fix visibility of global symbols in SWH plugins.

noisicaa/node_lib/swh/audio.cpp.tmpl

@@ -33,10 +33,12 @@
 #define buffer_write(b, v) (b = v)
 #define LADSPA_Data float
 
-{{ headerCode }}
-
 namespace noisicaa::engine::swh {
 
+namespace {
+{{ headerCode }}
+}
+
 class {{ className }} : public ProcessorNode {
 public:
   {{ className }}(EngineImpl* engine, uint64_t id)