COMP: Fix build error related to missing curl/curl.h

This commit fixes build error introduced by commit r23355 by moving the use
of "curl/curl.h" into the private implementation. That way, it won't be
required for user of the "vtkHTTPHandler" class to know about CURL, it
will be an implementation details.

Build error were occurring only windows because curl wasn't installed on the
system and the header wasn't found in default location.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23357 3bd1e089-480b-0410-8dfb-8563597acbee