imagemagick: needs thread support

The no thread support in imagemagick is broken since the last
bump. Since supporting non-threaded configurations is not that
important, let's simply make imagemagick depend on thread support.

Fixes:

  http://autobuild.buildroot.org/results/4e6/4e6d0bbe120579ab4a54960ee794e8cf64b99e80/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>