Revert "bpfilter: Fix build error with CONFIG_BPFILTER_UMH"

This reverts commit 1d273fcc.

Alexei points out there's nothing implying headers will be built
and therefore exist under usr/include, so this fix doesn't make
much sense.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/bpfilter/Makefile

@@ -5,7 +5,7 @@
 
 userprogs := bpfilter_umh
 bpfilter_umh-objs := main.o
-userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi -I usr/include/
+userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi
 
 ifeq ($(CONFIG_BPFILTER_UMH), y)
 # builtin bpfilter_umh should be linked with -static