Fix failure of --jobs with vacuumdb on Windows

FD_SETSIZE needs to be declared before winsock2.h, or it is possible to
run into buffer overflow issues when using --jobs.  This is similar to
pgbench's solution done in a23c641.

This has been introduced by 71d84ef, and older versions have been using
the default value of FD_SETSIZE, defined at 64.

Per buildfarm member jacana, but this impacts all Windows animals
running the TAP tests.  I have reproduced the failure locally to check
the patch.

Author: Michael Paquier
Reviewed-by: Andrew Dunstan
Discussion: https://postgr.es/m/20190826054000.GE7005@paquier.xyz
Backpatch-through: 9.5

src/bin/scripts/vacuumdb.c

@@ -10,6 +10,10 @@
  *-------------------------------------------------------------------------
  */
 
+#ifdef WIN32
+#define FD_SETSIZE 1024			/* must set before winsock2.h is included */
+#endif
+
 #include "postgres_fe.h"
 
 #ifdef HAVE_SYS_SELECT_H