Fix ext/snmp for newer net-snmp versions on Windows (GH-15888)

As of net-snmp 5.8.0, the library defines their own `(v)asprintf()` if
not available on the system.  However, PHP also does this, so when
building ext/snmp there would be conflicting declarations on Windows.
To avoid this, we explictly define `HAVE_ASPRINTF`, so net-snmp does
not redeclare when its headers are included.

ext/snmp/snmp.c

@@ -26,6 +26,11 @@
 #include "php.h"
 #include "main/php_network.h"
 #include "ext/standard/info.h"
+
+#ifdef PHP_WIN32
+// avoid conflicting declarations of (v)asprintf()
+# define HAVE_ASPRINTF
+#endif
 #include "php_snmp.h"
 
 #include "zend_exceptions.h"