package/mtd: test programs need threads

The test programs depend on pthreads:
http://git.infradead.org/mtd-utils.git/blob/1bfee8660131fca7a18f68e9548a18ca6b3378a0:/configure.ac#l43

Also propagate the threads dependency to BR2_PACKAGE_MTD_INTEGCK.

Fixes
http://autobuild.buildroot.net/results/879/879c79e505f65387a46c4be263dc8783c8ca61bf/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Arnout: add depends on BR2_USE_MMU to comment]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>