cdt: MSBuild: define GVDLL to ensure correct storage-class attributes
An upcoming commit will change the storage-class attributes to be
controlled by the GVDLL symbol in cdt.h.
Without this change, the following error would occur:
LINK : fatal error LNK1104: cannot open file 'C:\GitLab-Runner\builds\magjac\graphviz\Debug\Graphviz\bin\cdt.lib' [C:\GitLab-Runner\builds\magjac\graphviz\lib\pathplan\Pathplan.vcxproj]
Towards https://gitlab.com/graphviz/graphviz/-/issues/2173.