qtconcurrent/generaterun: adapt to earlier changes in qtconcurrentrun.h

Except for the license header and the new auto return functions, the
qtconcurrentrun.h header is now reproduced faithfully again.

Change-Id: Ib3b1ec10e75ddfb8e4bd8f7d765ce9d2fc1fc4f9
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>