Only declare stacks in headers

Don't define stacks in C source files: it causes warnings
about unused functions in some compilers.

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/include/internal/dane.h b/include/internal/dane.h
index cbe33f3..0d35cc2 100644
--- a/include/internal/dane.h
+++ b/include/internal/dane.h
@@ -97,6 +97,8 @@
     EVP_PKEY *spki;
 } danetls_record;
 
+DEFINE_STACK_OF(danetls_record)
+
 /*
  * Shared DANE context
  */