[sw/silicon_creator] Add missing initializer in romextimage unit test

Signed-off-by: Alphan Ulusoy <alphan@google.com>
diff --git a/sw/device/silicon_creator/mask_rom/romextimage_unittest.cc b/sw/device/silicon_creator/mask_rom/romextimage_unittest.cc
index 82c41b7..725a222 100644
--- a/sw/device/silicon_creator/mask_rom/romextimage_unittest.cc
+++ b/sw/device/silicon_creator/mask_rom/romextimage_unittest.cc
@@ -16,7 +16,7 @@
 class RomExtImage : public mask_rom_test::MaskRomTest {
  protected:
   mask_rom_test::MockRomextimagePtrs romextimage_ptrs_;
-  manifest_t manifest_;
+  manifest_t manifest_{};
 };
 
 TEST_F(RomExtImage, ManifestGet) {