samples: net: echo-client: Fix compile warning

This is similar fix as in commit 643cc4a22c but this one fixes
the thread parameter in UDP DTLS thread.

Fixes this function parameter datatype warning:

expected ‘k_thread_stack_t * {aka struct _k_thread_stack_element *}’
 but argument is of type ‘u8_t * {aka unsigned char *}’

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
1 file changed