Add generated benchmark files to gitignore

When building IntelliJ generates several source files related to the
benchmarks. This commit adds that path to the gitignore so these don't
get accidentally committed.

.gitignore

@@ -5,6 +5,8 @@
 *.ipr
 *.iws
 build-idea/
+# These files are generated in the main tree by IntelliJ
+benchmarks/src/main/generated/*
 
 # eclipse files
 .project
@@ -42,7 +44,7 @@ html_docs
 /tmp/
 eclipse-build
 
-# projects using testfixtures 
+# projects using testfixtures
 testfixtures_shared/
 
 # These are generated from .ci/jobs.t