Add missing header guards (#41322)
These keep turning up, so I did a little bash-fu to find them all.
```bash
grep -lL "#pragma once" $(grep -lL "#ifndef .*_H_" $(find . | grep "\.h$")) | cut -c 3-
```
diff --git a/common/constants.h b/common/constants.h
index 50b0f91..378f7c8 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -2,6 +2,11 @@
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
+#ifndef FLUTTER_COMMON_CONSTANTS_H_
+#define FLUTTER_COMMON_CONSTANTS_H_
+
namespace flutter {
constexpr double kMegaByteSizeInBytes = (1 << 20);
} // namespace flutter
+
+#endif // FLUTTER_COMMON_CONSTANTS_H_
