kernel: Fix _K_QUEUE_INITIALIZER portability issue.

_K_QUEUE_INITIALIZER macro provides initialisation for k_queue struct,
which contains an anonymous union.

Older versions of GCC (<= 4.5), even when compiling with -std=gnu99,
do not allow specifying members of an anonymous union without braces
in an initialiser, so it is necessary to add braces around anonymous
union members.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>