selftests/ir: fix build with ancient kernel headers

Since commit e2bcbd7769ee ("tools headers UAPI: remove stale lirc.h"),
the build of the selftests fails on rhel 8 since its version of
/usr/include/linux/lirc.h has no definition of RC_PROTO_RCMM32, etc [1].

[1] https://lkml.org/lkml/2022/1/28/275

Fixes: e2bcbd7769ee ("tools headers UAPI: remove stale lirc.h")
Reviewed-by: Shuah Khan <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Sean Young <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ir/ir_loopback.c

···
 #define SYSFS_PATH_MAX 256
 #define DNAME_PATH_MAX 256
 
+/*
+ * Support ancient lirc.h which does not have these values. Can be removed
+ * once RHEL 8 is no longer a relevant testing platform.
+ */
+#if RC_PROTO_MAX < 26
+#define RC_PROTO_RCMM12 24
+#define RC_PROTO_RCMM24 25
+#define RC_PROTO_RCMM32 26
+#endif
+
 static const struct {
 	enum rc_proto proto;
 	const char *name;