LibJS+LibJIT: Fix GCC build

Author: https://github.com/awesomekling
Commit: 8c745ca223
Pull-request: https://github.com/SerenityOS/serenity/pull/21619
Reviewed-by: https://github.com/Hendiadyoin1

Userland/Libraries/LibJIT/Assembler.h

@@ -228,7 +228,10 @@ struct Assembler {
 
     [[nodiscard]] Label make_label()
     {
-        return { .offset_of_label_in_instruction_stream = m_output.size() };
+        return Label {
+            .offset_of_label_in_instruction_stream = m_output.size(),
+            .jump_slot_offsets_in_instruction_stream = {},
+        };
     }
 
     [[nodiscard]] Label jump()