Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.

This should allow many algorithms to take advantage of more parallelization
technologies.

Affected functions surf.ocl, pyrlk.ocl and hog.ocl are updated with the change.

This reverts commit fba72cb6, reversing
changes made to 02131ffb.

Backport from master.