progress meter: add disabled code for a more stable ETA

Compiles to ~25 bytes if enabled.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -1949,6 +1949,7 @@ typedef struct bb_progress_t {
 	unsigned last_update_sec;
 	unsigned last_change_sec;
 	unsigned start_sec;
+	/*unsigned last_eta;*/
 	const char *curfile;
 } bb_progress_t;