kernel: add 'static' keyword to select routines

Applies the 'static' keyword to the following inlined routines:
    z_priq_dumb_add()
    z_priq_mq_add()
    z_priq_mq_remove()
As those routines are only used in one place, they no longer have
externally visible declarations.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>