access/parallel.h lacks PGDLLIMPORT

Hi hackers,

I suppose that extensions are supposed to be allowed to use the
facilities in access/parallel.h.  I noticed in passing when I wrote a
throwaway test harness that my Windows built drone complained:

test_sharedtuplestore.obj : error LNK2001: unresolved external symbol
ParallelWorkerNumber
[C:\projects\postgres\test_sharedtuplestore.vcxproj]
.\Release\test_sharedtuplestore\test_sharedtuplestore.dll : fatal
error LNK1120: 1 unresolved externals
[C:\projects\postgres\test_sharedtuplestore.vcxproj]

I suppose that all three of these might need that, if they're part of
the API for parallel worker management:

extern volatile bool ParallelMessagePending;
extern int  ParallelWorkerNumber;
extern bool InitializingParallelWorker;

I'm less sure about the other two but at least ParallelWorkerNumber is
essential for anything that needs to coordinate access to input/output
arrays or similar.

-- 
Thomas Munro
http://www.enterprisedb.com