[libcxx] Fix a missed case needing "system header" markings for init_priority(100)

[mstorsjo]

Summary

This was missed in D95972 / 7c49052b170f76f19be64a5572d31ad8f5df4e61,
as this codepath isn't exercised by CI yet.

FWIW, I should be able to add the necessary things to the windows
premerge runner image now, so we hopefully can add CI for the mingw
configuration soon. (Just waiting for some other updates to bundle
up more updates to the runner image at once, when botering someone to
rebuild it.)

libcxx/src/chrono.cpp

GetSystemTimeInit() {
     fp = (GetSystemTimeAsFileTimePtr)GetProcAddress(
         GetModuleHandleW(L"kernel32.dll"), "GetSystemTimePreciseAsFileTime");
     if (fp == nullptr)
       fp = GetSystemTimeAsFileTime;
   }
   GetSystemTimeAsFileTimePtr fp;
 };
 
+# 83 "chrono.cpp" 1 3
 GetSystemTimeInit GetSystemTimeAsFileTimeFunc _LIBCPP_INIT_PRIORITY_MAX;
+# 85 "chrono.cpp" 2
 } // namespace
 
 #endif
 
 static system_clock::time_point __libcpp_system_clock_now() {
   // FILETIME is in 100ns units
   using filetime_duration =
       _VSTD::chrono::duration<__int64,