Add .mailmap entries for jgruber

Change-Id: I67b6594d7ec5226d54f09954210fe4f6ec4d47b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3810172Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Auto-Submit: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82189}

.mailmap

@@ -11,4 +11,6 @@
 # Please keep the list sorted.
 
 Clemens Backes <clemensb@chromium.org> Clemens Hammacher <clemensh@chromium.org>
+Jakob Linke <jgruber@chromium.org>
+Jakob Linke <jgruber@chromium.org> <jgruber@google.com>
 Timothy Gu <timothygu@chromium.org> <timothygu99@gmail.com>