Put back <float.h> in a few files that need it for _isnan().

Further fallout from commit c29aff959: there are some files that need
<float.h>, and were getting it from datatype/timestamp.h, but it was not
apparent in my (tgl's) testing because the requirement for <float.h>
exists only on certain Windows toolchains.

Report and patch by David Rowley.

Discussion: https://postgr.es/m/CAKJS1f-BHceaFzZScFapDV48gUVM2CAOBfhkgffdqXzFb+kwew@mail.gmail.com

src/backend/access/gist/gistproc.c

@@ -17,6 +17,7 @@
  */
 #include "postgres.h"
 
+#include <float.h>
 #include <math.h>
 
 #include "access/gist.h"

src/backend/access/gist/gistutil.c

@@ -13,6 +13,7 @@
  */
 #include "postgres.h"
 
+#include <float.h>
 #include <math.h>
 
 #include "access/gist_private.h"