ortho: MSBuild: define GVC_EXPORTS to ensure correct storage-class attributes

ortho: MSBuild: define GVC_EXPORTS to ensure correct storage-class attributes

The ortho lib is a static library which is linked into the
dynamic-link gvc lib and both use GVC_EXPORTS to control the
storage-class.

Removes many warnings like:

LINK : warning LNK4286: symbol '_gcalloc' defined in 'memory.obj' is imported by 'gvortho.lib(ortho.obj)' [C:\Users\magja\graphviz\lib\gvc.vcxproj]
LINK : warning LNK4217: symbol '_gcalloc' defined in 'memory.obj' is imported by 'gvortho.lib(trapezoid.obj)' in function '_construct_trapezoids' [C:\Users\magja\graphviz\lib\gvc.vcxproj]
C:\Users\magja\graphviz\lib\ortho\ortho.c(1250,1): warning C4273: 'orthoEdges': inconsistent dll linkage [C:\Users\magja\graphviz\lib\ortho\gvortho.vcxproj]

Towards https://gitlab.com/graphviz/graphviz/-/issues/2058.