gitignore: ignore local APK keys

Those keys should never be added to the git repository.

Signed-off-by: Paul Spooren <mail@aparcar.org>

.gitignore

@@ -21,6 +21,8 @@
 /*.patch
 /llvm-bpf*
 key-build*
+private-key.pem
+public-key.pem
 *.orig
 *.rej
 *~