tools/include/linux/kernel.h and kselftest_harness.h are missing
ifndef guard around ARRAY_SIZE define. Fix them to avoid duplicate
define errors during compile when another file defines it. This
problem was found when compiling selftests that include a header
with ARRAY_SIZE define.

ARRAY_SIZE is defined in several selftests. There are about 25+
duplicate defines in various selftests source and header files.
Add ARRAY_SIZE to kselftest.h in preparation for removing duplicate
ARRAY_SIZE defines from individual test files.
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>