Fix help message for new configure option --enable-debug.

The new option is --enable-debug, not --disable-debug.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -608,7 +608,7 @@ echo "  --install=INSTALL        use specified install [$install]"
 echo "  --static                 enable static build [$static]"
 echo "  --enable-debug-tcg       enable TCG debugging"
 echo "  --disable-debug-tcg      disable TCG debugging (default)"
-echo "  --disable-debug          enable common debug build options"
+echo "  --enable-debug           enable common debug build options"
 echo "  --enable-sparse          enable sparse checker"
 echo "  --disable-sparse         disable sparse checker (default)"
 echo "  --disable-strip          disable stripping binaries"