dist/tools/headerguards: ignore if #include_next is used

We can't use the same header guard if we include two files with the same name.

dist/tools/headerguards/headerguards.py

@@ -43,6 +43,7 @@ def fix_headerguard(filename):
     tmp.seek(0)
 
     guard_found = 0
+    include_next_found = 0
     guard_name = ""
     ifstack = 0
     for line in inlines:
@@ -66,14 +67,17 @@ def fix_headerguard(filename):
                 else:
                     guard_found += 1
                     line = "#endif /* %s */\n" % supposed
+            elif line.startswith("#include_next"):
+                include_next_found = 1
 
         tmp.write(line)
 
     tmp.seek(0)
     if guard_found == 3:
-        for line in difflib.unified_diff(inlines, tmp.readlines(),
-                                         "%s" % filename, "%s" % filename):
-            sys.stdout.write(line)
+        if include_next_found == 0:
+            for line in difflib.unified_diff(inlines, tmp.readlines(),
+                                             "%s" % filename, "%s" % filename):
+                sys.stdout.write(line)
     else:
         print("%s: no / broken header guard" % filename, file=sys.stderr)
         return False