COMP: Fix function is not a member of std (Win)
The merge request !117 (merged) to add custom event handlers
for vtk interaction introduced a bug on Windows, where
the header needs to be included to be able
to use std::function.
This fixes the 50 errors listed on cdash (due to those
std::function members being used frequently):
http://my.cdash.org/viewBuildError.php?buildid=1111691
PS: The experimental build for this MR did not show any issues on CDash [1], but they appeared once merged in the master branch (continuous). There seem to be an issue with how buildbot runs those builds on Windows, since a similar issue happened on windows 32 bits with Eigen aligned data structures in stl containers.