libosmocore: Add dependency for libmnl

Since libosmocore.git 53a2fde368d0161a38bc096d121ef61a9afbbe94,
libosmocore depends by default on libmnl.
Let's make sure to depend on it in the recipe to avoid recipe build
breakage.

Change-Id: I62221706b010766af5eed77a08693e877a197c7f
1 file changed