library: refactor the XTRA_PROCPS_DEBUG implementation

If we ever were to eliminate the procps.h header file,
as discussed in the thread referenced below, then that
would impair the current XTRA_PROCPS_DEBUG provisions.

The only remaining way to verify result types would be
to explicitly include that <proc/xtra-procps-debug.h>.

So, this commit will once again enable the ./configure
provision for defining the -DXTRA_PROCPS_DEBUG option.

Reference(s):
https://www.freelists.org/post/procps/newlib-Qualys-patches,6

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

proc/diskstats.h

@@ -126,6 +126,10 @@ struct diskstats_stack **procps_diskstats_sort (
     enum diskstats_item sortitem,
     enum diskstats_sort_order order);
 
+
+#ifdef XTRA_PROCPS_DEBUG
+# include <proc/xtra-procps-debug.h>
+#endif
 #ifdef __cplusplus
 }
 #endif