Issue #3080: Mark _PyImport_FindBuiltin() argument as constant

And as a consequence, mark also name argument of
_PyImport_FindExtensionUnicode() constant too. But I plan to change this
argument type to PyObject* later.
diff --git a/Include/import.h b/Include/import.h
index 400e97c..26e1f0f 100644
--- a/Include/import.h
+++ b/Include/import.h
@@ -65,9 +65,9 @@
 PyAPI_FUNC(void) _PyImport_ReInitLock(void);
 
 PyAPI_FUNC(PyObject *)_PyImport_FindBuiltin(
-    char *name                  /* UTF-8 encoded string */
+    const char *name            /* UTF-8 encoded string */
     );
-PyAPI_FUNC(PyObject *)_PyImport_FindExtensionUnicode(char *, PyObject *);
+PyAPI_FUNC(PyObject *)_PyImport_FindExtensionUnicode(const char *, PyObject *);
 PyAPI_FUNC(int)_PyImport_FixupBuiltin(
     PyObject *mod,
     char *name                  /* UTF-8 encoded string */