Bug 1525638 - Let hg ignore the build directories of WebRender standalone builds. r=ted

Differential Revision: https://phabricator.services.mozilla.com/D18852

--HG--
extra : moz-landing-system : lando

.hgignore

@@ -190,3 +190,6 @@ tps_result\.json
 
 # Ignore file generated by lalrpop at build time.
 ^third_party/rust/lalrpop/src/parser/lrgrammar.rs
+
+# Ignore the build directories of WebRender standalone builds.
+gfx/wr/target/