selftests/capabilities: clean up for Makefile

Clean up the following things:
1.  Avoid the broken when use TARGETS in the command line, eg:

 $ make -C tools/testing/selftests TARGETS=capabilities
 make[1]: *** No rule to make target 'capabilities', needed by 'all'.  Stop.

    Replace TARGETS with BINARIES.

2.  User need to provide cap-ng.h and libcap-ng.so for cross compiling.
    Replace ':=' with '+=' for CFLAGS and introduce LDLIBS to archieve
    it. Delete useless EXTRA_CLAGS at the same time.

3.  Delete the duplicated definition which is already defined by
    lib.mk.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>