selftests/ftrace: install test.d

The ftrace test requires the directory test.d and all of it's contents to be
present during execution. Use TEST_DIRS to ensure this is copied to the
INSTALL_PATH.
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/ftrace/Makefile

 all:
 
 TEST_PROGS := ftracetest
+TEST_DIRS := test.d/
 
 include ../lib.mk