Changing to correct trace function invocaton

[btwardow]

With out correction running point-wise example:

addpath(genpath('src/'))
addpath(genpath('libs/'))
source('src/scripts/demo_gppw.m')

will result ending with error message:

error: 'traceAB' undefined near line 85 column 25
error: called from:
error:   /Users/bartlomiej.twardowsk/Development/github/gpfm-master/src/pairwise/gppwInf.m at line 85, column 17
error:   /Users/bartlomiej.twardowsk/Development/github/gpfm-master/src/pairwise/gppwSgdLearn.m at line 40, column 16
error:   /Users/bartlomiej.twardowsk/Development/github/gpfm-master/src/scripts/demo_gppw.m at line 33, column 7