simde-diagnostic: Include simde-arch
simde-diagnostic: Include simde-arch

When checking for the SIMDE_DIAGNOSTIC_DISABLE_VECTOR_CONVERSION_
simde-arch.h was getting included after simde-diagnostic.h
This caused SIMDE_ARCH_ARM to not be defined during the check.

nemequ/simde,nemequ/simde,nemequ/simde