libdrgn/python: unexport set_drgn_error()

This was previously used for testing internals via ctypes, but it's no
longer needed.

Fixes: 7d251fe ("Translate C lexer tests to C unit tests")
Signed-off-by: Omar Sandoval <osandov@osandov.com>

libdrgn/python/error.c

@@ -117,7 +117,7 @@ struct drgn_error *drgn_error_from_python(void)
 	return err;
 }
 
-DRGNPY_PUBLIC void *set_drgn_error(struct drgn_error *err)
+void *set_drgn_error(struct drgn_error *err)
 {
 	if (err == &drgn_error_python)
 		return NULL;