Ignore infer output folder

Infer [1] is a static code checker.

[1] http://fbinfer.com/

Change-Id: I880cefe0a20f6af88ab10f6e862fda44fbe0883d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

.gitignore

@@ -3,3 +3,4 @@
 /buck-cache
 /buck-out
 /target
+infer-out