ntpd: fix build with -fno-common

Only a small nit here: psl should be declared extern and defined exactly
once.

-fno-common will become the default in GCC10/LLVM11.

MFC after:	3 days

contrib/ntp/include/ntp_config.h

@@ -280,7 +280,7 @@ typedef struct settrap_parms_tag {
  * Poll Skew List
  */
 
-psl_item psl[17-3+1];		/* values for polls 3-17 */
+extern psl_item psl[17-3+1];	/* values for polls 3-17 */
 				/* To simplify the runtime code we */
 				/* don't want to have to special-case */
 				/* dealing with a default */

contrib/ntp/ntpd/ntp_config.c

@@ -202,6 +202,8 @@ int	cryptosw;		/* crypto command called */
 
 extern char *stats_drift_file;	/* name of the driftfile */
 
+psl_item psl[17-3+1];
+
 #ifdef BC_LIST_FRAMEWORK_NOT_YET_USED
 /*
  * backwards compatibility flags