drivers: slip: remove unneeded include path addition

subdir-ccflags-y +=-I${srctree}/subsys/net/ip isn't need so lets
remove it.

Change-Id: I60d97ce25398d7d3801e837075dbf75d1375e055
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
diff --git a/drivers/slip/Makefile b/drivers/slip/Makefile
index ebc2e24..69c2cee 100644
--- a/drivers/slip/Makefile
+++ b/drivers/slip/Makefile
@@ -1,3 +1 @@
-subdir-ccflags-y +=-I${srctree}/subsys/net/ip
-
 obj-$(CONFIG_SLIP) = slip.o