selftests: Fix wrong TARGET in kselftest top level Makefile

The 'uevents' subdirectory does not exist in tools/testing/selftests/
and adding 'uevents' to the TARGETS list results in the following error:

make[1]: Entering directory 'xx/tools/testing/selftests/uevents'
make[1]: *** No targets specified and no makefile found. Stop.
make[1]: Leaving directory 'xx/tools/testing/selftests/uevents'

What actually exists in tools/testing/selftests/ is the 'uevent'
subdirectory.

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>