[Tracker] single quotes for arguments to convert, svn r778

["Marcus Fritzsch" <fritschy googlemail com>]

Hi,

i noted that he single quotes introduces with svn r778 are causing an
oddness when parsing the output of the extractor, i.e. single quotes
in the width field value.

As g_spawn_async does not runs it's commands with /bin/sh (as
system(3)) these quotes aren't necessary and therefore are part of the
output format.

No patch as this is a rather trivial change, see
tracker-extract-imagemagick.c:46 ;)

Cheers, Marcus