src/meta-openembedded:freediameter: fix dependency from libidn to lib…

…idn2

* it's used since:
  freeDiameter/freeDiameter@da679d2
  included with the upgrade to 1.5.0 in:
  https://git.openembedded.org/meta-openembedded/commit/?id=47ccb88d94852e327f3bdd45425f33e56983b50c

* libidn2 is usually pulled into the RSS by the dependency from gnutls
  but when gnutls doesn't depend on it, the build fails with:
-- Checking for module 'libidn2'
--   No package 'libidn2' found
CMake Error at include/freeDiameter/CMakeLists.txt:144 (MESSAGE):
  Unable to find libidn2, please install libidn2-dev or equivalent, or set
  DIAMID_IDNA_IGNORE or DIAMID_IDNA_REJECT

Signed-off-by: Martin Jansa <[email protected]>
Signed-off-by: Khem Raj <[email protected]>