gitignore: add service file

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ib144b0759ce3b5db92e16c8801df365fb07e1a7c
diff --git a/.gitignore b/.gitignore
index a11e810..e60573c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -46,3 +46,5 @@
 *_unittest.log
 *_unittest.trs
 test-suite.log
+
+phosphor-pid-control.service