size-stats: fix code style

Fix these warnings:
E302 expected 2 blank lines, found 1
E305 expected 2 blank lines after class or function definition, found 1
E713 test for membership should be 'not in'

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>