testcases/kernel/include/lib.mk: fix invalid libkerntest.a path

In testcases/kernel/include/lib.mk, '$(KERNEL_DIR)/libkerntest.a' is not a
valid path, the correct path is '$(LIBKERNTEST_DIR)/libkerntest.a'.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>