Fix moc stumbling over gcc __attribute__ extensions

Fix moc stumbling over gcc __attribute__ extensions

Reported by David Faure.

In KDE a DEPRECATED macro gets defined in a header file created by cmake.
The define is not guarded with #if Q_CC_GNU or similar because at cmake
time the compiler is determined. Therefore moc suddenly sees this gcc
specific token and stumbles over it.

This patch simply defines an empty __attribute__ macro that will expand
to nothing and thus become invisible to moc's "C++ parser" after the
pre-processing.

Change-Id: I4448b9ac3f72b6334e32b27484401fb0fca23a0c
Reviewed-by: David Faure <faure@kde.org>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>