Include VectorClass's instrset.h in TestVDTMath tests for INSTRSET

Currently all TestVDTMathXYZ tests are broken, giving.

 > TestVDTMathSSE4.exe 
CPU level 8 does not support compilation level 999. Test will not be run.

Appears to have happened sometime ago, when the build stopped including the vectorclass header that defines the INSTRSET macro. This adds it back.