gfio: bad merge in Makefile

Inadverdently dropped lib/prio_tree.c, which we use for graph
tooltips.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/Makefile b/Makefile
index a521220..367c19a 100644
--- a/Makefile
+++ b/Makefile
@@ -20,8 +20,8 @@
 		lib/num2str.c lib/ieee754.c $(wildcard crc/*.c) engines/cpu.c \
 		engines/mmap.c engines/sync.c engines/null.c engines/net.c \
 		memalign.c server.c client.c iolog.c backend.c libfio.c flow.c \
-		cconv.c json.c lib/zipf.c lib/axmap.c lib/lfsr.c \
-		gettime-thread.c
+		cconv.c lib/prio_tree.c json.c lib/zipf.c lib/axmap.c \
+		lib/lfsr.c gettime-thread.c
 
 ifeq ($(UNAME), Linux)
   SOURCE += diskutil.c fifo.c blktrace.c helpers.c cgroup.c trim.c \