Revert DownloadEngine.cc change in 6b55f5d3 because Debian Lenny breaks

src/DownloadEngine.cc

@@ -34,13 +34,7 @@
 /* copyright --> */
 #include "DownloadEngine.h"
 
-#ifdef HAVE_SYS_SIGNAL_H
-#  include <sys/signal.h>
-#else // HAVE_SYS_SIGNAL_H
-#  ifdef HAVE_SIGNAL_H
-#    include <signal.h>
-#  endif // HAVE_SIGNAL_H
-#endif // HAVE_SYS_SIGNAL_H
+#include <signal.h>
 
 #include <cstring>
 #include <cerrno>