Doxyfile.in: adding some excludes for lwip related files
Doxyfile.in: adding some excludes for lwip related files

Due changes to the *.c and *.h files (added for the lwip support) we have
to exclude files that are not related to the library.

gebart/libcoap,authmillenon/libcoap,gebart/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap