BUILD: listener: shut up a build warning when threads are disabled

We get this with __decl_hathreads due to the lone semi-colon, let's move
it at the end of the innermost declaration :

  src/listener.c: In function 'listener_accept':
  src/listener.c:601:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
1 file changed