third_party: Update mbedtls/BUILD.gn to be a relative path
third_party: Update mbedtls/BUILD.gn to be a relative path

Updating to be a relative path instead of a absolute path.

Bug: b/239560853
Test: Verified with RBE enabled and disabled
Change-Id: Ia6d0b7cec41bd35a887ec81d84d6b31a7207434d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/107969
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Reviewed-by: Joe Brennan <fa94e96f73b3491ccd33472b6aea764155c95752@google.com>
Pigweed-Auto-Submit: Joe Brennan <fa94e96f73b3491ccd33472b6aea764155c95752@google.com>
Commit-Queue: Joe Brennan <fa94e96f73b3491ccd33472b6aea764155c95752@google.com>

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