Fix function prototype generation for unused but public/api entries

Cython/Compiler/ModuleNode.py

@@ -915,7 +915,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
 
     def generate_cfunction_declarations(self, env, code, definition):
         for entry in env.cfunc_entries:
-            if entry.used:
+            if entry.used or (entry.visibility == 'public' or entry.api):
                 generate_cfunction_declaration(entry, env, code, definition)
 
     def generate_variable_definitions(self, env, code):