Correct MSVC storage-class in gvpr.h. Removes one warning

Correct MSVC storage-class in gvpr.h. Removes one warning

Only set __declspec(dllexport) or __declspec(dllimport) when the gvpr
library is dynamically linked. This is defined by GVDLL which is set
in the Windows MSBuild builds which use dynamic linking, but unset in
the Windows CMake builds which use static linking.

Removes the following warning from the Windows CMake builds:

C:\GitLab-Runner\builds\graphviz\graphviz\lib\gvpr\gvpr.c(931,1): warning C4273: 'gvpr': inconsistent dll linkage [C:\GitLab-Runner\builds\graphviz\graphviz\build\lib\gvpr\gvpr.vcxproj]