library: rename 'diskstat' source as 'diskstats' files

Where possible, libprocps files convey the name of the
actual source pseudo file under the '/proc' directory.

This brings diskstats into line with such an approach.

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/procps.h

@@ -19,7 +19,7 @@
 #define PROCPS_PROC_PROCPS_H
 
 /* includes that show public exports go here */
-#include <proc/diskstat.h>
+#include <proc/diskstats.h>
 #include <proc/meminfo.h>
 #include <proc/namespace.h>
 #include <proc/pids.h>