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.

doc/Doxyfile.in

@@ -789,7 +789,8 @@ RECURSIVE              = NO
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                =
+EXCLUDE                = @top_srcdir@/src/coap_io_lwip.c \
+                         @top_srcdir@/include/coap/lwippools.h
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded