pgraphnodes: Add missing ShaderGenerator stub symbols

panda/src/pgraphnodes/shaderGenerator.h

@@ -197,8 +197,8 @@ private:
 // If we don't have Cg, let's replace this with a stub.
 class EXPCL_PANDA_PGRAPHNODES ShaderGenerator : public TypedReferenceCount {
 PUBLISHED:
-  ShaderGenerator(const GraphicsStateGuardianBase *gsg);
-  virtual ~ShaderGenerator();
+  ShaderGenerator(const GraphicsStateGuardianBase *gsg) {}
+  virtual ~ShaderGenerator() {}
 
   virtual CPT(ShaderAttrib) synthesize_shader(const RenderState *rs,
                                               const GeomVertexAnimationSpec &anim) {