Include missing src/common/fd.h header in src/slurmctld/front_end.c.

Front-end builds wouldn't compile otherwise. Oversight of 3f947d45.

Bug 7146.

NEWS

@@ -28,6 +28,7 @@ documents those changes that are of interest to users and administrators.
  -- select/cons_res - fix debug flag SelectType handling in select_p_job_test.
  -- Fix sacctmgr archive dump commit confirmation.
  -- Prevent extra resources from being allocated when combining certain flags.
+ -- Fix build with --enable-front-end configure option.
 * Changes in Slurm 19.05.0
 ==========================

src/slurmctld/front_end.c

@@ -45,6 +45,7 @@
 #include <unistd.h>
 
 #include "slurm/slurm.h"
+#include "src/common/fd.h"
 #include "src/common/list.h"
 #include "src/common/log.h"
 #include "src/common/node_conf.h"