Put the doxygen dependency on the quickbook file, rather than boostbook.

Because when building the main documentation, the docbook target isn't
being triggered, which is breaking the build. Hopefully this will fix
it.

doc/Jamfile.v2

@@ -24,14 +24,13 @@ doxygen autodoc
         <xsl:param>"boost.doxygen.reftitle=Boost.TypeIndex Header Reference"
    ;
 
-xml type_index : type_index.qbk ;
+xml type_index : type_index.qbk : <dependency>autodoc ;
 boostbook standalone
     :
         type_index
     :
         <xsl:param>boost.root=http://www.boost.org/doc/libs/1_53_0
 #        <xsl:param>boost.root=../../../..
         <format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/doc/html
-        <dependency>autodoc
     ;