BUILD/MINOR: Makefile : enabling USE_CPU_AFFINITY

FreeBSD can handle cpuset matters just fine, we can hence enable it
by default as linux2628 TARGET.
diff --git a/Makefile b/Makefile
index 26b55db..2acf502 100644
--- a/Makefile
+++ b/Makefile
@@ -324,6 +324,7 @@
   USE_TPROXY     = implicit
   USE_LIBCRYPT   = implicit
   USE_THREAD     = implicit
+  USE_CPU_AFFINITY= implicit
 else
 ifeq ($(TARGET),osx)
   # This is for Mac OS/X