Replace CONFIG_CPU_COUNT with plain CPU_COUNT

Fixes double-declartion issue of CPU_COUNT when fio.h is included by
third-party projects.

Checking for a define of CPU_COUNT seems to be a common approach
to deal with different versions of sched.h

Signed-off-by: Daniel Gollub <daniel.gollub@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/os/os.h b/os/os.h
index 2f2d069..98b4532 100644
--- a/os/os.h
+++ b/os/os.h
@@ -321,7 +321,7 @@
 }
 #endif
 
-#ifndef CONFIG_CPU_COUNT
+#ifndef CPU_COUNT
 #ifdef FIO_HAVE_CPU_AFFINITY
 static inline int CPU_COUNT(os_cpu_mask_t *mask)
 {