RFR (trivial): 8197827: [windows] Small cleanups after JDK-8185712

[Thomas Stüfe]

Hi all,

could I have reviews and a sponsor please for this very trivial
windows-only change.

Just a cleanup up some remnants of "JDK-8185712 [windows] Improve native
symbol decoder" - deleting an orphaned function prototype not needed
anymore, correcting include guard names and adding a missing include.

Bug: https://bugs.openjdk.java.net/browse/JDK-8197827
Webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8197827-cleanups-windows/
webrev.00/webrev/

Thanks, Thomas