xmlto: set CLEANBROKEN = "1"

xmlto: set CLEANBROKEN = "1"

While rebuilding xmlto or xmlto-native, `make clean' in do_configure
will remove man/man1/xmlif.1 which triger failure in do_compile

$ bitbake xmlto-native
$ bitbake xmlto-native -cconfigure -f
$ bitbake xmlto-native -ccompile -f
...
|FORMAT_DIR=./format /bin/bash ./xmlto -o man/man1 man doc/xmlif.xml
|xmlto: tmp-glibc/work/x86_64-linux/xmlto-native/0.0.28-r0/xmlto-0.0.28/
doc/xmlif.xml does not validate (status 3)
|xmlto: Fix document syntax or use --skip-validation option
I/O error : Attempt to load network entity http://www.oasis-open.org/
docbook/xml/4.2/docbookx.dtd
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>