selftests/sgx: Add "test_encl.elf" to TEST_FILES

The "test_encl.elf" file used by test_sgx is not installed in
INSTALL_PATH. Attempting to execute test_sgx causes false negative:

"
enclave executable open(): No such file or directory
main.c:188:unclobbered_vdso:Failed to load the test enclave.
"

Add "test_encl.elf" to TEST_FILES so that it will be installed.

Fixes: 2adcba79

 ("selftests/x86: Add a selftest for SGX")
Signed-off-by: Yi Lai <yi1.lai@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>