add gitignore

Just like many other OpenBMC repos, this commit adds the gitignore file.

Also manually removed one indirect dependency wrap file.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I751fc6398824d88a2a98341fbea822cc461aa728
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..b208e37
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+build*/
+subprojects/*
+!subprojects/*.wrap
+subprojects/phosphor-dbus-interfaces.wrap