Fix format in platform plugin loading error reporting.

qguiapplication.cpp:792:133: warning: format not a string literal and
no format arguments [-Wformat-security].

Introduced  by 0d7d53fd46e8ab512bed88d0dab114cfada227ce .

Change-Id: I70268a8b597be33226efb9bb4d7d9f672f0d7440
Reviewed-by: Kai Koehne <kai.koehne@digia.com>