Have git ignore files in third_party/eigen3/src

https://chromium-review.googlesource.com/c/chromium/src/+/2523426
adds things to third_party/eigen3 , not third_party/eigen, so this was
most likely a typo.

Bug: 1121333
Change-Id: I2cd34e88026142df479d7573dbf63cecf8f0d501
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2549301Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829158}

third_party/.gitignore

@@ -69,7 +69,7 @@
 /devtools-frontend/src
 /directxsdk
 /dom_distiller_js/dist
-/eigen/src
+/eigen3/src
 /elfutils/src
 /emoji-segmenter/src
 /espresso/lib/