Speed up fixincludes.

In my case:
$ rm ./stmp-fixinc ; time make -j16

takes 17 seconds, where I can reduce it easily with the suggested
change. Then I get to 11.2 seconds.

The scripts searches ~2500 folders in my case with total 20K header
files.

fixincludes/ChangeLog:

	* fixinc.in: Use mkdir -p rather that a loop.