library: add delta values with swap too, <meminfo> api

As an oversight, delta values for SWAP amounts weren't
included in the <meminfo> API. Since any runtime costs
of including them only amount to slightly more storage
this commit will simply correct the earlier oversight.

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

proc/meminfo.h

@@ -126,7 +126,12 @@ enum meminfo_item {
     MEMINFO_SWAP_CACHED,           //  ul_int
     MEMINFO_SWAP_FREE,             //  ul_int
     MEMINFO_SWAP_TOTAL,            //  ul_int
-    MEMINFO_SWAP_USED              //  ul_int
+    MEMINFO_SWAP_USED,             //  ul_int
+
+    MEMINFO_SWAP_DELTA_CACHED,     //   s_int
+    MEMINFO_SWAP_DELTA_FREE,       //   s_int
+    MEMINFO_SWAP_DELTA_TOTAL,      //   s_int
+    MEMINFO_SWAP_DELTA_USED        //   s_int
 };