Fix build

Half.hpp now requires <cmath>

Change-Id: Iab086681cfd3ed850ad184c77f88d13b5716e859
Reviewed-on: https://swiftshader-review.googlesource.com/c/23688
Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/System/Half.hpp b/src/System/Half.hpp
index 5c23dca..a20629a 100644
--- a/src/System/Half.hpp
+++ b/src/System/Half.hpp
@@ -16,6 +16,7 @@
 #define sw_Half_hpp
 
 #include <algorithm>
+#include <cmath>
 
 namespace sw
 {