Chances are, your kernel is installed as /usr/src/linux-2.x.yy rather than /usr/src/linux/ . Try this:
locate include | grep linux | less
Your terminal will probably barf up a lot of results, thus the pipe to less in the example above
You could also try looking in the /usr/src directory to see what directories live there, too:
ls -l /usr/src | grep linux | grep "^d"
On my system, this returns:
$ls -l /usr/src | grep linux | grep "^d"
drwxr-xr-x 15 573 573 4096 Feb 21 04:38 linux-2.4.28
If I look in the linux-2.4.28 directory, I see an include directory there:
ls -l /usr/src/linux-2.4.28/
drwxr-xr-x 29 573 573 4096 Feb 3 05:11 include