CSS: Don't reference images that aren't generated by Doxygen (#2295)

Signed-off-by: Jan Stephan <jan.stephan@amd.com>

projects/hip/docs/doxygen/Doxyfile

@@ -1243,7 +1243,8 @@ HTML_STYLESHEET        =
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  = ../_doxygen/extra_stylesheet.css
+HTML_EXTRA_STYLESHEET  = ../_doxygen/extra_stylesheet.css \
+                         ../_doxygen/doxygen-img-ref-fix.css
 
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note