pw_chrono: Remove unnecessary parentheses

The parentheses around pw_chrono_SystemClock_Duration sometimes
trigger -Wc99-extensions warnings since the statement is interpreted as
a compound literal.

Change-Id: I0b1d8a4a6e32fc549054ba96c09e7df067ff171c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/96180
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

pw_chrono/public/pw_chrono/internal/system_clock_macros.h

@@ -14,7 +14,7 @@
 #pragma once
 
 #define _PW_SYSTEM_CLOCK_DURATION(num_ticks) \
-  ((pw_chrono_SystemClock_Duration){.ticks = (num_ticks)})
+  (pw_chrono_SystemClock_Duration{.ticks = (num_ticks)})
 
 // clang-format off