Include <math.h> for isnan.  This avoids an implicit function
declaration and a build failure with future compilers.

Submitted upstream: <https://github.com/postgis/postgis/pull/725>

diff -ur postgis-3.3.2.orig/loader/shp2pgsql-core.c postgis-3.3.2/loader/shp2pgsql-core.c
--- postgis-3.3.2.orig/loader/shp2pgsql-core.c	2022-11-13 08:09:23.000000000 +0100
+++ postgis-3.3.2/loader/shp2pgsql-core.c	2023-04-09 22:01:46.377934865 +0200
@@ -15,6 +15,8 @@
 
 #include "../postgis_config.h"
 
+#include <math.h> /* for isnan */
+
 #include "shp2pgsql-core.h"
 #include "../liblwgeom/liblwgeom.h"
 #include "../liblwgeom/lwgeom_log.h" /* for LWDEBUG macros */