kbuild: finally remove the obsolete variable $TOPDIR

TOPDIR is obsolete, it can be finally removed now.

Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/drivers/scsi/cxgb3i/Kbuild b/drivers/scsi/cxgb3i/Kbuild
index 25a2032..70d060b 100644
--- a/drivers/scsi/cxgb3i/Kbuild
+++ b/drivers/scsi/cxgb3i/Kbuild
@@ -1,4 +1,4 @@
-EXTRA_CFLAGS += -I$(TOPDIR)/drivers/net/cxgb3
+EXTRA_CFLAGS += -I$(srctree)/drivers/net/cxgb3
 
 cxgb3i-y := cxgb3i_init.o cxgb3i_iscsi.o cxgb3i_pdu.o cxgb3i_offload.o cxgb3i_ddp.o
 obj-$(CONFIG_SCSI_CXGB3_ISCSI) += cxgb3i.o