cleanup-mounts-under: Don't fail when /proc/mounts isn't readable

cleanup-mounts-under: Don't fail when /proc/mounts isn't readable

Not all operating systems provide a "/proc/mounts" file (for exmaple,
Darwin/OS X doesn't), so ignore this error for now ...

This doesn't solve the problem per se but makes "make clean" usable
on those systems again.

Signed-off-by: Alexander Barton <alex@barton.de>