WORKSPACE: Move platforms dep to the top
WORKSPACE: Move platforms dep to the top

Otherwise, the version specified here will be ignored if `platforms` is
a transitive dep of one of our other external dependencies.

Bug: 258835053
Change-Id: I12b03401c674092fc2b2bc0defe4f11982250172
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/118932
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>
Pigweed-Auto-Submit: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed