BUG/MINOR: threads: don't drop "extern" on the lock in include files

Commit 9dcf9b6 ("MINOR: threads: Use __decl_hathreads to declare locks")
accidently lost a few "extern" in certain lock declarations, possibly
causing certain entries to be declared at multiple places. Apparently
it hasn't caused any harm though.

The offending ones were :
  - fdtab_lock
  - fdcache_lock
  - poll_lock
  - buffer_wq_lock
diff --git a/include/proto/fd.h b/include/proto/fd.h
index f81c646..ce23c1c 100644
--- a/include/proto/fd.h
+++ b/include/proto/fd.h
@@ -39,9 +39,9 @@
 extern THREAD_LOCAL int *fd_updt;  // FD updates list
 extern THREAD_LOCAL int fd_nbupdt; // number of updates in the list
 
-__decl_hathreads(HA_SPINLOCK_T fdtab_lock);      /* global lock to protect fdtab array */
-__decl_hathreads(HA_RWLOCK_T   fdcache_lock);    /* global lock to protect fd_cache array */
-__decl_hathreads(HA_SPINLOCK_T poll_lock);       /* global lock to protect poll info */
+__decl_hathreads(extern HA_SPINLOCK_T fdtab_lock);      /* global lock to protect fdtab array */
+__decl_hathreads(extern HA_RWLOCK_T   fdcache_lock);    /* global lock to protect fd_cache array */
+__decl_hathreads(extern HA_SPINLOCK_T poll_lock);       /* global lock to protect poll info */
 
 /* Deletes an FD from the fdsets, and recomputes the maxfd limit.
  * The file descriptor is also closed.