explicitly define underlying enum type

[samuelpmishLLNL]

on CUDA11 w/ MSVC 2019, I get a lot of warnings when using trove:

[build] F:\fem_performance_tests\extern\trove\trove/warp.h(42): warning: integer conversion resulted in a change of sign
[build] 
[build] F:\fem_performance_tests\extern\trove\trove/shfl.h(53): warning: integer conversion resulted in a change of sign
[build] 
[build] F:\fem_performance_tests\extern\trove\trove/transpose.h(421): warning: integer conversion resulted in a change of sign
.
.
.

They seem to be addressed by manually specifying the underlying type for the enum in warp.h.