BUILD: checks: fix inlining issue on set_srv_agent_[addr,port}

BUILD: checks: fix inlining issue on set_srv_agent_[addr,port}

These functions are declared as external functions in check.h and
as inline functions in check.c. Let's move them as static inline in
check.h. This appeared in 2.4 with the following commits:

  4858fb2e1 ("MEDIUM: check: align agentaddr and agentport behaviour")
  1c921cd74 ("BUG/MINOR: check: consitent way to set agentaddr")

While harmless (it only triggers build warnings with some gcc 4.x),
it should probably be backported where the paches above are present
to keep the code consistent.