libsyncml: move build dependencies from RRECOMMENDS to DEPENDS

Putting build dependencies into RRECOMMENDS isn't going to work, so move them to
DEPENDS.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>