selftests/mount_setattr: fix to make run_tests failure

make run_tests doesn't run the test. Fix Makefile to set TEST_GEN_PROGS
instead of TEST_GEN_FILES to fix the problem.

run_tests runs TEST_GEN_PROGS, TEST_CUSTOM_PROGS, and TEST_PROGS.
TEST_GEN_FILES is for files generated by tests.
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/mount_setattr/Makefile

@@ -2,6 +2,6 @@
 # Makefile for mount selftests.
 CFLAGS = -g $(KHDR_INCLUDES) -Wall -O2 -pthread
 
-TEST_GEN_FILES += mount_setattr_test
+TEST_GEN_PROGS := mount_setattr_test
 
 include ../lib.mk