base: Fix the compiler guard in base/pollevent.hh

This was not formatted consistently with other files.

Change-Id: Ibee20cb6f458fcf99a1729cb8bd50c9c536d6639
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45740
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/pollevent.hh

@@ -26,8 +26,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef __POLLEVENT_H__
-#define __POLLEVENT_H__
+#ifndef __BASE_POLLEVENT_HH__
+#define __BASE_POLLEVENT_HH__
 
 #include <poll.h>
 
@@ -110,4 +110,4 @@ class PollQueue
  */
 extern PollQueue pollQueue;
 
-#endif // __POLLEVENT_H__
+#endif // __BASE_POLLEVENT_HH__