mksquashfsimage.sh: Add forward slash for SRC_DIR

There is a bug in mksquashfs that fails to interpret symlink dirs unless
they have a forward slash

Change-Id: I64df07d99858a792e1d7af5d11234fe8b75f91ae
Signed-off-by: Mohamad Ayyash <mkayyash@google.com>
1 file changed