swift-backtrace: Fix a timespec initialization warning.

NFC.

Author: tshortli

stdlib/public/libexec/swift-backtrace/main.swift

@@ -551,7 +551,7 @@ Generate a backtrace for the parent process.
           // If the output path is a directory, generate a filename
           let name = target!.name
           let pid = target!.pid
-          var now = timespec()
+          var now = timespec(tv_sec: 0, tv_nsec: 0)
 
           if clock_gettime(CLOCK_REALTIME, &now) != 0 {
             now.tv_sec = time(nil)