selftests: Use lirc.h from kernel tree, not from system

When the system lirc.h is older than v4.16, you will get errors like:

ir_loopback.c:32:16: error: field ‘proto’ has incomplete type
  enum rc_proto proto;

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: NSean Young <sean@mess.org>
Signed-off-by: NShuah Khan <shuah@kernel.org>

tools/testing/selftests/ir/Makefile

 # SPDX-License-Identifier: GPL-2.0
 TEST_PROGS := ir_loopback.sh
 TEST_GEN_PROGS_EXTENDED := ir_loopback
+APIDIR := ../../../include/uapi
+CFLAGS += -Wall -O2 -I$(APIDIR)
 
 include ../lib.mk