package/mono: add threads dependency

This patch adds a dependency on BR2_TOOLCHAIN_HAS_THREADS to the mono
package, in order to fix the following autobuilder failures:

  http://autobuild.buildroot.net/results/ece/ece1051cb9238735130b29f76d90a4994058345a/
  http://autobuild.buildroot.net/results/658/65850774d4c6b875cb6ebbf461dcc48cf8208f4f/
  http://autobuild.buildroot.net/results/5d5/5d5074aa73e167099410d244a18114dbced348d1/



Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>