bpfilter: re-add header search paths to tools include to fix build error

I thought header search paths to tools/include(/uapi) were unneeded,
but it looks like a build error occurs depending on the compiler.

Commit 303a339f ("bpfilter: remove extra header search paths for
bpfilter_umh") reintroduced the build error fixed by commit ae40832e
("bpfilter: fix a build err").

Apology for the breakage, and thanks to Guenter for reporting this.

Fixes: 303a339f ("bpfilter: remove extra header search paths for bpfilter_umh")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bpfilter/Makefile

@@ -5,6 +5,7 @@
 
 hostprogs-y := bpfilter_umh
 bpfilter_umh-objs := main.o
+KBUILD_HOSTCFLAGS += -Itools/include/ -Itools/include/uapi
 HOSTCC := $(CC)
 
 ifeq ($(CONFIG_BPFILTER_UMH), y)