fix PlatformToolSet tag location in vcxproj files

The PlatformToolSet tag belongs into the PropertyGroup with the label
"Configuration". The former location in an anonymous PropertyGroup
tricked Visual Studio into displaying the right PlatformToolSet but
using its default value. If VS 2010 and VS 2012 are freshly installed
on the same machine, the default toolset for VS 2012 is VS 2010.

Task-number: QTBUG-30822

Change-Id: If00a532e92b0812c552b1cac52ff77a1e7039146
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from commit 853a0b764e08d5f910072b6f3fd3ff3f9dc5c0c4)