Add a missing cast in rotation.h

Thanks to Lukas Post (@deutschepost) for reporting this.

https://github.com/ceres-solver/ceres-solver/issues/497

Change-Id: I6d9d0ca5feab3a1f4cf47613537e8cd7ab1ba479

include/ceres/rotation.h

diff --git a/include/ceres/rotation.h b/include/ceres/rotation.h
index 8ba58ab..3020a2d 100644
--- a/include/ceres/rotation.h
+++ b/include/ceres/rotation.h
@@ -307,7 +307,7 @@
     //              = atan(-sin(theta), -cos(theta))
     //
     const T two_theta =
-        T(2.0) * ((cos_theta < 0.0)
+        T(2.0) * ((cos_theta < T(0.0))
                   ? atan2(-sin_theta, -cos_theta)
                   : atan2(sin_theta, cos_theta));
     const T k = two_theta / sin_theta;