system/Clock: compile only on Windows

src/system/meson.build

@@ -1,6 +1,5 @@
 system_sources = [
   'EventPipe.cxx',
-  'Clock.cxx',
 ]
 
 if host_machine.system() == 'linux'
@@ -12,6 +11,12 @@ if host_machine.system() == 'linux'
   ]
 endif
 
+if host_machine.system() == 'windows'
+  system_sources += [
+    'Clock.cxx',
+  ]
+endif
+
 system = static_library(
   'system',
   system_sources,