py/obj: Fix mp_obj_is_type compilation with C++.

Fixes issue #12951.

Signed-off-by: stijn <stijn@ignitron.net>

examples/usercmodule/cppexample/example.cpp

@@ -1,9 +1,16 @@
 extern "C" {
 #include <examplemodule.h>
+#include <py/objstr.h>
 
 // Here we implement the function using C++ code, but since it's
 // declaration has to be compatible with C everything goes in extern "C" scope.
 mp_obj_t cppfunc(mp_obj_t a_obj, mp_obj_t b_obj) {
+    // The following no-ops are just here to verify the static assertions used in
+    // the public API all compile with C++.
+    MP_STATIC_ASSERT_STR_ARRAY_COMPATIBLE;
+    if (mp_obj_is_type(a_obj, &mp_type_BaseException)) {
+    }
+
     // Prove we have (at least) C++11 features.
     const auto a = mp_obj_get_int(a_obj);
     const auto b = mp_obj_get_int(b_obj);