Half.hpp now requires <cmath>

Change-Id: Iab086681cfd3ed850ad184c77f88d13b5716e859
Reviewed-on: https://swiftshader-review.googlesource.com/c/23688Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>