Make QtLocation compile with pre-compiled headers

For some mystical reason Polygon is redefined by an unknown Windows
header preventing this file from compiling if Qt is compiled
with pre-compiled headers.

The workaround is to simply rename this private type.

Discussed-in: http://lists.qt-project.org/pipermail/development/2014-May/017125.html
Task-number: QTBUG-39692
Change-Id: If71e1a12394dfde7729d0505417a37b9fd407c82
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>