count: Include atomic_nehalem.pdf instead of atomic_nehalem.png

Without any suffix specified, \includegraphics{} prefers .png to .pdf
by default.

Fixes: 04e270767c34 ("count: Restore 'fig:count:Atomic Increment Scalability on Nehalem'")
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>