py/makeqstrdefs.py: Use python 2.6 syntax for set creation.

py/makeqstrdefs.py declares that it works with python 2.6 however the
syntax used to initialise of a set with values was only added in python
2.7. This leads to build failures when the host system doesn't have
python 2.7 or newer.

Instead of using the new syntax pass a list of initial values through
set() to achieve the same result. This should work for python versions
from at least 2.6 onwards.

Helped-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Chris Packham <judge.packham@gmail.com>