Add RTC_EXPORT to webrtc::Clock

Bug: chromium:1011581
Change-Id: I827ccff6946d3c86af67adcd00b57fad0440489f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170043
Commit-Queue: Johannes Kron <kron@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30748}
diff --git a/system_wrappers/BUILD.gn b/system_wrappers/BUILD.gn
index 9130739..1ff2ddd 100644
--- a/system_wrappers/BUILD.gn
+++ b/system_wrappers/BUILD.gn
@@ -37,6 +37,7 @@
     "../rtc_base:checks",
     "../rtc_base/synchronization:rw_lock_wrapper",
     "../rtc_base/system:arch",
+    "../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
 
diff --git a/system_wrappers/include/clock.h b/system_wrappers/include/clock.h
index 94c8746..8eac3d1 100644
--- a/system_wrappers/include/clock.h
+++ b/system_wrappers/include/clock.h
@@ -17,6 +17,7 @@
 
 #include "api/units/timestamp.h"
 #include "rtc_base/synchronization/rw_lock_wrapper.h"
+#include "rtc_base/system/rtc_export.h"
 #include "system_wrappers/include/ntp_time.h"
 
 namespace webrtc {
@@ -28,7 +29,7 @@
 const double kMagicNtpFractionalUnit = 4.294967296E+9;
 
 // A clock interface that allows reading of absolute and relative timestamps.
-class Clock {
+class RTC_EXPORT Clock {
  public:
   virtual ~Clock() {}
   // Return a timestamp relative to an unspecified epoch.