samples: add .gitignore for pidfd-metadata

Ignore the pidfd-metadata binary so it doesn't show up in unwanted
scenarios.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner <christian@brauner.io>
diff --git a/samples/pidfd/.gitignore b/samples/pidfd/.gitignore
new file mode 100644
index 0000000..be52b3b
--- /dev/null
+++ b/samples/pidfd/.gitignore
@@ -0,0 +1 @@
+pidfd-metadata