Add third_party/swiftshader to gitignore

As of 7d27602 swiftshader is pulled in through
DEPS. Add the swiftshader directory to the gitignore list to keep the files
from showing up as untracked.

Review-Url: https://codereview.chromium.org/2223103003
Cr-Commit-Position: refs/heads/master@{#410401}

.gitignore

@@ -433,7 +433,7 @@ vs-chromium-project.txt
 /third_party/sqlite4java/lib/**/*.jar
 /third_party/sqlite4java/lib/**/*.jnilib
 /third_party/sqlite4java/lib/**/*.so
-/third_party/swiftshader/include/
+/third_party/swiftshader/
 /third_party/syzygy
 /third_party/syzygy/binaries
 /third_party/tsan/