[fuchsia] Update to new spelling of magenta syscalls

runtime/vm/os_thread_fuchsia.cc

Index: runtime/vm/os_thread_fuchsia.cc
diff --git a/runtime/vm/os_thread_fuchsia.cc b/runtime/vm/os_thread_fuchsia.cc
index c3e8d25173152579227bde779fa1c3fc798b53ec..cf8970dd610ef9d47e24990c4dad08ae50c7ba0c 100644
--- a/runtime/vm/os_thread_fuchsia.cc
+++ b/runtime/vm/os_thread_fuchsia.cc
@@ -47,7 +47,7 @@ namespace dart {
 
 static void ComputeTimeSpecMicros(struct timespec* ts, int64_t micros) {
   // time in nanoseconds.
-  mx_time_t now = _magenta_current_time();
+  mx_time_t now = mx_current_time();
   mx_time_t target = now + (micros * kNanosecondsPerMicrosecond);
   int64_t secs = target / kNanosecondsPerSecond;
   int64_t nanos = target - (secs * kNanosecondsPerSecond);