Disable USE_PDFLATEX in doxyfile to enable build of graphs via dot

* with this option enabled, dot is called with -Tpdf which is not
  supported by the current build options in the graphviz spack
  package

Change-Id: I8e755d3bc86ca00585bd7f96bba7a456ec568779

doxyfile

@@ -1713,7 +1713,7 @@ PDF_HYPERLINKS         = YES
 # The default value is: YES.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
-USE_PDFLATEX           = YES
+USE_PDFLATEX           = NO
 
 # If the LATEX_BATCHMODE tag is set to YES, doxygen will add the \batchmode
 # command to the generated LaTeX files. This will instruct LaTeX to keep running