psmisc: do not install pstree.x11 symbolic link

The psmisc-no-__progname.patch removes the usage of __progname, which
removes any difference between pstree and pstree.x11. Therefore, we
make sure to not create the pstree.x11 symbolic link in the target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>