modphp: use libdir instead of baselib to load module

When we change the apache2 files layout to debian style,
the ServerRoot in httpd.conf was changed to "/" from "/usr",
then the relative path to libphp5 module in 70_mod_php5.conf
will be invalid so it fails to load the module, use libdir
(absolute path) instead so it will always find the module.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>