Windows: Fix typo: eglContext

Use eglContext instead of eglcontext as a parameter of
nativeResourceForContext() function on windows:
qtbase/src/plugins/platforms/windows/qwindowsnativeinterface.cpp

Change-Id: Id6b4b5b6b8befad5b8a20b44e1c2a80c034921f9
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>