[libcxx] [test] Add an XFAIL for the timespec test for MinGW targets

MinGW headers/libs lack timespec_get.

Differential Revision: https://reviews.llvm.org/D112213

Author: mstorsjo

libcxx/test/std/language.support/support.runtime/ctime.timespec.compile.pass.cpp

@@ -15,6 +15,9 @@
 // unavailable until macOS 10.15
 // XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12|13|14}}
 
+// MinGW lacks timespec_get.
+// XFAIL: target={{.+}}-windows-gnu
+
 #include <ctime>
 #include <type_traits>