Enable message d_groupId field regardless of build

We currently do some cleverness with where and when we enable the
incomplete message group ID feature in `CMakeLists.txt`.  However, when
using the `build-darwin.sh` and `build-ubuntu.sh` scripts, as we
currently recommend, the libraries we install using `make install` have
this feature enabled.

Unfortunately, enabling message group IDs changes the size of
`bmqa_message`’s `MessageImpl`, so a library built with message group
IDs enabled is ABI-incompatible with an application built with message
group IDs disabled.  Because we don’t advertise this feature anywhere or
export the definition from CMake, users are unlikely to know they need
to compile with the flag `-DBMQ_ENABLE_MSG_GROUPID`, and will run into
characteristic ABI-incompatibility issues: buffer overruns, stack
clobbering, and segmentation faults.

This patch makes a minimal change until we decide what to do with
message group IDs: the API is still sensitive to
`BMQ_ENABLE_MSG_GROUPID`, but the message structure itself always
carries a `d_groupId` field, regardless of its presence.  This field is
default-initialized and is otherwise unused.

Signed-off-by: Patrick M. Niedzielski <[email protected]>

src/groups/bmq/bmqa/bmqa_message.h

@@ -121,11 +121,11 @@ struct MessageImpl {
     // SubscriptionHandle this message is
     // associated with
 
-#ifdef BMQ_ENABLE_MSG_GROUPID
+    // #ifdef BMQ_ENABLE_MSG_GROUPID
     bsl::string d_groupId;
     // Optional Group Id this message is
     // associated with
-#endif
+    // #endif
 };
 
 // ===============================