Remove bogus "extern" annotations on function definitions.

While this is not illegal C, project style is to put "extern" only on
declarations not definitions.

David Rowley

Discussion: https://postgr.es/m/CAKJS1f9RKLWXcMBQhvDYhmsMEo+ALuNgA-NE+AX5Uoke9DJ2Xg@mail.gmail.com

src/backend/foreign/foreign.c

@@ -712,7 +712,7 @@ get_foreign_server_oid(const char *servername, bool missing_ok)
  * path list in RelOptInfo is anyway sorted by total cost we are likely to
  * choose the most efficient path, which is all for the best.
  */
-extern Path *
+Path *
 GetExistingLocalJoinPath(RelOptInfo *joinrel)
 {
 	ListCell   *lc;