Fix violation of the one definition rule in ext/com_dotnet (GH-15327)

The definition of the class entries in the internal header file is not
correct, since that file is included several times, and even the
comment above the definition hints at com_extension.c where the actual
definition is.  We fix this by declaring these variables as `extern`.

Author: cmb69

ext/com_dotnet/php_com_dotnet_internal.h

@@ -66,7 +66,7 @@ static inline bool php_com_is_valid_object(zval *zv)
 } while(0)
 
 /* com_extension.c */
-zend_class_entry *php_com_variant_class_entry, *php_com_exception_class_entry, *php_com_saproxy_class_entry;
+extern zend_class_entry *php_com_variant_class_entry, *php_com_exception_class_entry, *php_com_saproxy_class_entry;
 
 /* com_handlers.c */
 zend_object* php_com_object_new(zend_class_entry *ce);