selftests: pidfd: Create .gitignore to include pidfd_test

Create ../selftests/pidfd/.gitignore which holds the following file name
created after compiling:

	- pidfd_test

Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
diff --git a/tools/testing/selftests/pidfd/.gitignore b/tools/testing/selftests/pidfd/.gitignore
new file mode 100644
index 0000000..822a1e6
--- /dev/null
+++ b/tools/testing/selftests/pidfd/.gitignore
@@ -0,0 +1 @@
+pidfd_test