[libc++] Add "using-if-exists" to timespec_get in modules

Picolibc does not provide timespec_get function. Adding
"using-if-exists" attribute fixes the modules.

Author: domin144

libcxx/modules/std.compat/ctime.inc

@@ -24,5 +24,5 @@ export {
   using ::mktime;
   using ::strftime;
   using ::time;
-  using ::timespec_get;
+  using ::timespec_get _LIBCPP_USING_IF_EXISTS;
 } // export

libcxx/modules/std/ctime.inc

@@ -24,5 +24,5 @@ export namespace std {
   using std::mktime;
   using std::strftime;
   using std::time;
-  using std::timespec_get;
+  using std::timespec_get _LIBCPP_USING_IF_EXISTS;
 } // namespace std

libcxx/test/std/modules/std.compat.pass.cpp

@@ -11,10 +11,6 @@
 // UNSUPPORTED: libcpp-has-no-std-modules
 // UNSUPPORTED: clang-modules-build
 
-// picolibc does not provide the required timespec_get function, and the
-// "using-if-exists" mechanism apparently did not work here.
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // A minimal test to validate import works.
 
 import std.compat;