nested_extend_c testcase fix when compiled by C++ target languages

Javascript v8 and node compiles the wrappers as C++, fix this warning:
anonymous type with no linkage used to declare variable ‘<anonymous struct> THING’ with linkage

Examples/test-suite/nested_extend_c.i

@@ -95,7 +95,7 @@ typedef struct {
     } bar;
 } FOO;
 
-struct {
+static struct {
    int i;
 } THING;
 %}