EMSUSD-1351 - MayaUsd: Code cleanup - No pragma once

[seando-adsk]

EMSUSD-1351 - MayaUsd: Code cleanup

• Adhere to include guard (instead of "#pragma once") from our coding standard.

[seando-adsk]

According to our coding standard:

Do not use #pragma once to avoid files being #include’d multiple times (which can cause duplication of definitions & compilation errors). While it's widely supported, it's a non-standard and non-portable extension.

Never use reserved identifiers, like underscore followed immediately by an uppercase letter or double underscore.