selftests/user_events: add a note about user_events.h dependency

This test depends on <linux/user_events.h> exported in uapi
The following commit removed user_events.h out of uapi:
commit 5cfff569 ("tracing: Move user_events.h temporarily out
   of include/uapi")

This test will not compile until user_events.h is added back to uapi.
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/user_events/Makefile

@@ -2,6 +2,14 @@
 CFLAGS += -Wl,-no-as-needed -Wall $(KHDR_INCLUDES)
 LDLIBS += -lrt -lpthread -lm
 
+# Note:
+# This test depends on <linux/user_events.h> exported in uapi
+# The following commit removed user_events.h out of uapi:
+# commit 5cfff569cab8bf544bab62c911c5d6efd5af5e05
+# tracing: Move user_events.h temporarily out of include/uapi
+# This test will not compile until user_events.h is added
+# back to uapi.
+
 TEST_GEN_PROGS = ftrace_test dyn_test perf_test
 
 TEST_FILES := settings