configure: show debug-info option in --help output

"--enable-debug-info" and "--disable-debug-info" were not shown
in --help output.
Signed-off-by: NDunrong Huang <huangdr@cloud-times.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

configure

@@ -1057,6 +1057,8 @@ echo "  --localstatedir=PATH     install local state in PATH"
 echo "  --with-confsuffix=SUFFIX suffix for QEMU data inside datadir and sysconfdir [$confsuffix]"
 echo "  --enable-debug-tcg       enable TCG debugging"
 echo "  --disable-debug-tcg      disable TCG debugging (default)"
+echo "  --enable-debug-info       enable debugging information (default)"
+echo "  --disable-debug-info      disable debugging information"
 echo "  --enable-debug           enable common debug build options"
 echo "  --enable-sparse          enable sparse checker"
 echo "  --disable-sparse         disable sparse checker (default)"