selftests: vm: update .gitignore with missing generated file

Add missing generated file to .gitignore.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/vm/.gitignore

@@ -10,3 +10,4 @@ userfaultfd
 mlock-intersect-test
 mlock-random-test
 virtual_address_range
+gup_benchmark