xproto: disable spec generation

xproto: disable spec generation

it's simply broken with below errors:

| /usr/bin/xmlto: line 343: echo: write error: Broken pipe
| Making portrait pages on letter paper (215.9mmx279.4mm)
| Making portrait pages on letter paper (215.9mmx279.4mm)
| [warning] /usr/bin/fop: Unable to locate servlet-api in /usr/share/java
| [warning] /usr/bin/fop: Unable to locate servlet-api in /usr/share/java
| Nov 29, 2010 8:42:16 AM org.apache.fop.apps.FOURIResolver resolve
| SEVERE: Error with opening URL 'http://docbook.sourceforge.net/release/images/draft.png': No route to host

So temporarily disable it until we have a better solution for doc generation later

Signed-off-by: Kevin Tian <kevin.tian@intel.com>