[yocto] prerequisite tools for making documentation

[jfabernathy]

I had a need to look at the bitbake manual in the 
poky/bitbake/doc/manual directory and tried to make pdf.  I had all the 
prerequisites listed in the QS guide.  However, I found the make failed 
due to a number of xml related tools being missing.  I installed those 
and got the pdf manual generated.

Should we add those xml tools to the prerequisites?  BTW, I'm using 
denzil branch and referring to the QS guide 1.2.

Jim A