Place the Qt plugin metadata in a PE-COFF section with MinGW too

Commit ec360d7a

 made it work for ELF
platforms, Apple platforms and for MSVC, but we apparently forgot it for
MinGW. This patch corrects that mistake.

We won't have the PE-COFF section parser until 5.5, but this will at
least making Qt 5.4-built plugins work on the faster case.

Change-Id: I51b06837dc321eaa4724c9598293cf85570f67fc
Reviewed-by: Lars Knoll <lars.knoll@digia.com>