[gn build] (manually) port 79f99ba

Author: nico

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -209,6 +209,7 @@ copy("include") {
     "support/musl/xlocale.h",
     "support/newlib/xlocale.h",
     "support/nuttx/xlocale.h",
+    "support/openbsd/xlocale.h",
     "support/solaris/floatingpoint.h",
     "support/solaris/wchar.h",
     "support/solaris/xlocale.h",