gvpack: use GVDLL instead of _WIN32 to enable dllimport storage-class attribute in...

gvpack: use GVDLL instead of _WIN32 to enable dllimport storage-class attribute in gvpack.c

Fixes these errors when building statically with MinGW:

C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: gvpack_static-gvpack.o: in function `main':
C:\Users\magja\graphviz\cmd\tools/gvpack.c:857: undefined reference to `__imp_gvplugin_neato_layout_LTX_library'
collect2.exe: error: ld returned 1 exit status
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: gvpack-gvpack.o: in function `main':
C:\Users\magja\graphviz\cmd\tools/gvpack.c:857: undefined reference to `__imp_gvplugin_neato_layout_LTX_library'

Fixes https://gitlab.com/graphviz/graphviz/-/issues/2173.