Move struct declaration out of the union to avoid a compiler warning.

Clang 3.3+ warns about this being an extension:
qsqlerror.h:101: warning: anonymous types declared in an anonymous union are an extension [-Wnested-anon-types]

Change-Id: I0350b977ff85558338b3b9db53d3fce7facb7635
Reviewed-by: Mark Brand <mabrand@mabrand.nl>

src/sql/kernel/qsqlerror.h

@@ -96,12 +96,13 @@ private:
     // ### Qt6: Keep the pointer and remove the rest.
     QString unused1;
     QString unused2;
+    struct Unused {
+        ErrorType unused3;
+        int unused4;
+    };
     union {
         QSqlErrorPrivate *d;
-        struct {
-            ErrorType unused3;
-            int unused4;
-        } unused5;
+        Unused unused5;
     };
 };