Add third_party/libc++static to .gitignore

This should have been done in https://codereview.webrtc.org/1475643010/

TBR=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1486733002 .

Cr-Commit-Position: refs/heads/master@{#10844}
diff --git a/.gitignore b/.gitignore
index 57bd245..51e2249 100644
--- a/.gitignore
+++ b/.gitignore
@@ -102,6 +102,7 @@
 /third_party/junit
 /third_party/junit-jar
 /third_party/libc++
+/third_party/libc++-static
 /third_party/libc++abi
 /third_party/libevent
 /third_party/libjingle