tests: __declspec (dll{ex,im}port) in tests/exceptions.at

tests: __declspec (dll{ex,im}port) in tests/exceptions.at

* tests/exceptions.at (common.h, module.h, lib.h) [w32]: Use
__declspec (dllimport) and __declspec (dllexport) instead of
the less portable __attribute__ ((dllimport)) and
__attribute__ ((dllexport)).  Makes the test compile on MSVC.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>