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 <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 TARGETS += tmpfs
 TARGETS += tpm2
 TARGETS += tty
-TARGETS += uevents
+TARGETS += uevent
 TARGETS += user
 TARGETS += user_events
 TARGETS += vDSO