mgmt: smp: add missing kernel.h include statement

There is following error when compiling applications using smp:

  /zephyr/include/mgmt/smp.h:77:16: error: field 'zst_work' has \
  incomplete type
     77 |  struct k_work zst_work;
        |                ^~~~~~~~
  /zephyr/include/mgmt/smp.h:80:16: error: field 'zst_fifo' has \
  incomplete type
     80 |  struct k_fifo zst_fifo;
        |                ^~~~~~~~

Fix that by adding missing kernel.h include statement in mgmt/smp.h.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
1 file changed