flann: disable on microblaze

All gcc versions fail to build flann on the Microblaze architecture, due
to gcc bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69401, so let's
disable this package on this architecture.

Fixes:

  http://autobuild.buildroot.net/results/3f44a1f30a88dbe4a3a83055267b472b58769e15/



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