fix 'make clean': shell glob, ignore failure

Unfortunately a glob like osmo-x__*.{svg,png} doesn't work, so have the
suffixes in separate globs.

Add dashes to indicate that failure should be ignored.

Change-Id: I6bc4d9ea72b43a573acbc860c23397f748de2c7b
1 file changed
tree: 290fe010f5a80db4b24fa6fdc5fb444dc88e77a0
  1. doc/