Add internal benchmarks to .gitignore.

Change-Id: Ic9f698084c123fd508f279cb1b7278466b41fbc9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152645
Reviewed-by: Clement Skau <cskau@google.com>
Commit-Queue: Clement Skau <cskau@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>

Author: sstrickl

.gitignore

@@ -3,6 +3,7 @@
 /.project
 /Makefile
 /base
+/benchmarks-internal
 /buildtools
 /ipch
 /out