Update .gitignore

Hi,

after running configure with "Enable developer documentation" set to
"yes" git status warns about two new untracked files:

        doc/doxygen/tooldev.doxygen
        doc/doxygen/tools.doxygen

Below is a small patch.

HTH,
  Dawid

.gitignore

@@ -34,6 +34,7 @@ cscope.out
 /depcomp
 /docs/developer
 /docs/tools
+/docs/doxygen/*.doxygen
 /install-sh
 /libtool
 /ltmain.sh