Gitignore doxygen output files in their new location

We moved doxygen documentation, so build artifacts are no longer being ignored. This commit sets .gitignore all documentation output in its new location

.gitignore

@@ -12,8 +12,7 @@
 *~
 obj_*
 Makefile.target
-doc/html
-doc/latex
+tools/doxygen/html
 patches-*
 tools/tunslip
 tools/tunslip6