Windows: Rename cm_btree struct data

Rename the "struct data" in cm_btree.h to something less generic in
order to avoid conflicts with other code.

Change-Id: I0580be084d508c195d767152af4f9a461cf0407e
Reviewed-on: http://gerrit.openafs.org/7898
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>

src/WINNT/afsd/cm_btree.h

@@ -78,7 +78,7 @@ typedef struct leaf {
     Nptr	nextNode;
 } Leaf;
 
-typedef struct data {
+typedef struct btree_data {
     keyT        key;
     dataT	value;
     Nptr        next;