Make event_debug_get_logging_mask_() real symbol (win32)

Make event_debug_get_logging_mask_() real symbol (win32)

I cannot made it work without this, even though dumpbin shows that that symbol
exists in the event_core.dll, event_extra.dll failed to compile:
==> win:      Creating library C:/vagrant/.cmake-vagrant/lib/Debug/event_extra.lib and object C:/vagrant/.cmake-vagrant/lib/Debug/event_extra.exp
==> win: http.obj : error LNK2001: unresolved external symbol _event_debug_logging_mask_ [C:\vagrant\.cmake-vagrant\event_extra_shared.vcxproj]
==> win: C:\vagrant\.cmake-vagrant\bin\Debug\event_extra.dll : fatal error LNK1120: 1 unresolved externals [C:\vagrant\.cmake-vagrant\event_extra_shared.vcxproj]
==> win: Done Building Project "C:\vagrant\.cmake-vagrant\event_extra_shared.vcxproj" (default targets) -- FAILED.

And dumpbin:
  sh-4.1$ /cygdrive/c/Program\ Files\ \(x86\)/Microsoft\ Visual\ Studio\ 12.0/VC/bin/amd64/dumpbin.exe /EXPORTS ./bin/Debug/event_core.dll
M
    202   C9 00059A3C event_debug_logging_mask_ = _event_debug_logging_mask_