Conditionalize hyperv support in gettimeofday(2) based on MK_HYPERV

The effect at runtime is negligible as the hyperv timer isn't available
except when hyperv is loaded.

This is a prerequisite for conditionalizing the header build/install out
of the build

MFC after: 3 weeks
Reviewed by: sephe
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9242