Update qgsproject.cpp

if projPath is like "D:/output/bin/release/../data/ykw/ykw.qgs"
(Please note the "../"),
and srcPath is like "D:/output/bin/data/ykw/ge.tif"
the relative path generated is wrong as "../../../../data/ykw/ge.tif"
so dir0.canonicalPath() will Returns the canonical path, i.e. a path without symbolic links or redundant "." or ".." elements.