Windows: Add _dclass to the allowed symbols list

We use isnan() and isinf() in JSON_ENC now, which is translated to a
call to Microsoft's standard library function _dclass.

.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23517)

util/platform_symbols/windows-symbols.txt

@@ -229,3 +229,4 @@ qsort
 _stat64i32
 atoi
 __stdio_common_vsprintf
+_dclass