mcumgr: select NET_BUF when MCUMGR=y

mcumgr is missing dependency on net_buf, which is used for SMP protocol
implementation. This causes build failure in case when only SMP over
shell is selected (CONFIG_MCUMGR_SMP_SHELL=y).

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