kernel32/tests: Fix compilation on systems that don't support nameless unions.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://newtestbot.winehq.org/JobDetails.pl?Key=2634

Your paranoid android.


=== w7pro64 (64 bit comm) ===
comm.c:861: Test failed: WaitCommEvent failed with a timeout
comm.c:872: recovering after WAIT_TIMEOUT...
comm.c:883: Test failed: WaitCommEvent error 0
comm.c:885: Test failed: WaitCommEvent: expected EV_TXEMPTY, got 0
comm.c:889: WaitCommEvent for EV_TXEMPTY took 1014 ms (timeout 1000)
comm.c:891: Test failed: WaitCommEvent used 1014 ms for waiting