LaTeX: Use \ul (from the "soul" package) instead of \uline

* ox-latex.el (org-latex-text-markup-alist): Use \ul{...} for
underlining instead of \uline{...} as requiring the hyperref
package makes \uline{...} unusable on headline.

* org.el (org-latex-default-packages-alist): Add the "soul"
package.

Thanks to Konstantin Kliakhandler for reporting this.

lisp/org.el

@@ -3843,6 +3843,7 @@ header, or they will be appended."
     (""     "wrapfig"   nil)
     (""     "rotating"  nil)
     ("normalem" "ulem"  t)
+    (""     "soul"      t)
     (""     "amsmath"   t)
     (""     "textcomp"  t)
     (""     "marvosym"  t)

lisp/ox-latex.el

@@ -533,7 +533,7 @@ When nil, no transformation is made."
 					 (code . verb)
 					 (italic . "\\emph{%s}")
 					 (strike-through . "\\sout{%s}")
-					 (underline . "\\uline{%s}")
+					 (underline . "\\ul{%s}")
 					 (verbatim . protectedtexttt))
   "Alist of LaTeX expressions to convert text markup.