[roll third_party/pigweed] pw_rpc: Switch PW_RPC_USE_GLOBAL_MUTEX to 1

Having locking disabled in RPC is a dangerous default.  For example:
pw_transfer depends on RPC being reentrant.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/94300

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 7339e260ae40f34..e2dcca1857dabf5
Roller-URL: https://ci.chromium.org/b/8814249380091857553
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8814249380091857553
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie7d921b24b0cdddee5d68286f5e897048e391d16
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/94553
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/third_party/pigweed b/third_party/pigweed
index 7339e26..e2dcca1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 7339e260ae40f34d80e9e0519129adf4c4a32ae8
+Subproject commit e2dcca1857dabf5d9ff3e6bb55f5e8e3e06d2924