hdGatling: fix compilation against latest USD dev #246

Job	Run time
Build MDL SDK / Build Ubuntu 20.04 / GCC	44m 22s
Build MDL SDK / Build Windows 10 / MSVC	42m 15s
Build gatling for USD v23.11 (Release) / Build USD v23.11 for Ubuntu / GCC (Release) / Build gatling	9m 15s
Build gatling for USD v23.11 (Release) / Build USD v23.11 for Ubuntu / clang (Release) / Build gatling	8m 43s
Build gatling for USD v23.11 (Release) / Build USD v23.11 for Windows / MSVC (Release) / Build gatling	16m 27s
Build gatling for USD v24.03 (Release) / Build USD v24.03 for Ubuntu / clang (Release) / Build gatling	8m 38s
Build gatling for USD v24.03 (Release) / Build USD v24.03 for Ubuntu / GCC (Release) / Build gatling	9m 14s
Build gatling for USD v24.03 (Release) / Build USD v24.03 for Windows / MSVC (Release) / Build gatling	9m 50s
Build gatling for USD v24.05 (Release) / Build USD v24.05 for Ubuntu / clang (Release) / Build gatling	9m 17s
Build gatling for USD v24.05 (Release) / Build USD v24.05 for Ubuntu / GCC (Release) / Build gatling	9m 3s
Build gatling for USD v24.05 (Release) / Build USD v24.05 for Windows / MSVC (Release) / Build gatling	16m 46s
	3h 3m 50s

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