Fix a compile warning in non-Linux case

There was no return value if it wasn't Linux.  Now returning "N/A"

Author: Beirdo

mythtv/libs/libmythtv/jitterometer.cpp

@@ -177,5 +177,7 @@ QString Jitterometer::GetCPUStat(void)
         ptr += 9;
     }
     return res;
+#else
+    return "N/A";
 #endif
 }