[roll pigweed] third_party: Update mbedtls/BUILD.gn to be a relative path

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

Test: Verified with RBE enabled and disabled
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/107969

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: fac7fe4de1373a5..372139347d6b2b7
Roller-URL: https://ci.chromium.org/b/8805001323193890209
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4aeb264d0de49e29a2766c63ff68d16bed323ca0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/107986
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pigweed b/pigweed
index fac7fe4..3721393 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit fac7fe4de1373a5617829ec2a5fa2305f2328ad4
+Subproject commit 372139347d6b2b7a1eee8c15237d519b2363a090