base: Tag API variables in date.cc

Change-Id: I5a90f6496ef1e39b6135a14dfdf3f5febd22ece2
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32974
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

src/base/date.cc

@@ -26,4 +26,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+/**
+ * @ingroup api_base_utils
+ */
 const char *compileDate = __DATE__ " " __TIME__;