cpu: include set in o3/commit_impl.

While the majority of compilers seemed to pickup set from else where,
one version of gcc 4.7 complains, so explictly add it.

src/cpu/o3/commit_impl.hh

@@ -42,6 +42,7 @@
  */
 
 #include <algorithm>
+#include <set>
 #include <string>
 
 #include "arch/utility.hh"