Fix ninja build warning on Linux.
Fix ninja build warning on Linux.

The 'all' target has a special meaning to ninja, and it would
complain about having two 'all' targets. Fix this by using a
capital 'a'.

BUG=None

Change-Id: I3ce2aab8df4bdfc8f49361f514881005547c51bf
Reviewed-on: https://chromium-review.googlesource.com/272258
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

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