Add third_party/soda to .gitignore

This CL adds the //third_party/soda/ path to .gitignore. This
third-party repo will be imported from src-internal/DEPS and contain
the files required to test the Speech On-Device API(SODA).

Bug: 1122784
Change-Id: I0513a157a83f72781e0db444acb7d5e6f0f43318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386459Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#806319}

third_party/.gitignore

@@ -207,6 +207,7 @@
 /skia
 /smhasher/src
 /snappy/src
+/soda
 /speex
 /spirv-cross/spirv-cross
 /spirv-headers/src