asn1t.h: silence -Wmissing-prototype in Windows builds.

On Windows OPENSSL_EXPORT_VAR_AS_FUNCTION is defined and in a sense
this modification simply harmonizes it with "VAR_AS_VAR".
Reviewed-by: NRich Salz <rsalz@openssl.org>

include/openssl/asn1t.h

@@ -103,7 +103,7 @@ extern "C" {
                 static const ASN1_ITEM local_it = {
 
 #  define static_ASN1_ITEM_start(itname) \
-        ASN1_ITEM_start(itname)
+        static ASN1_ITEM_start(itname)
 
 #  define ASN1_ITEM_end(itname) \
                 }; \