[Plplot-general] dll export of some functions in plstream.h

[Phil R. <p.d...@gm...>]

Hi All
I've just made a commit exporting a couple of functions into the
plplotcxx dll and import lib (I'm building on Windows, commit
223dff54af4cd13d12c50b26ceb8f8aa67dc6f84). This change fixes a linker
error I was getting.

I'm only just really starting to make use of dlls and import libs
rather than just using static libs, so if anyone wishes to check out
the changes and tell me I've done something incorrectly then feel free
to let me know and I can look at changing things again.

Phil