Fix the #include path for ICU headers

The ICU headers in os_fuchsia.cc have been included with their path
relative to the dart root directory.  They should instead be included
relative to the paths mentioned in `-I` directives of the current
compiler invocation.

This allows downstreams that have a different directory organization to
build dart sdk even if they don't exactly replicate the directory
structure of dart sdk.

Change-Id: Iec79f261aee5ccc7b74448978fc69244e260400a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154163
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Filip Filmar <fmil@google.com>

Author: filmil

runtime/vm/os_fuchsia.cc

@@ -21,8 +21,8 @@
 #include <zircon/syscalls/object.h>
 #include <zircon/types.h>
 
-#include "third_party/icu/source/common/unicode/errorcode.h"
-#include "third_party/icu/source/i18n/unicode/timezone.h"
+#include "unicode/errorcode.h"
+#include "unicode/timezone.h"
 
 #include "platform/assert.h"
 #include "platform/syslog.h"