[libc++abi] Guard include of <unistd.h> behind __has_include

This doesn't change anything on platforms that have <unistd.h>, but
it will allow this file to compile on platforms that do not.

libcxxabi/src/cxa_guard_impl.h

@@ -39,11 +39,13 @@
 
 #include "__cxxabi_config.h"
 #include "include/atomic_support.h" // from libc++
-#include <unistd.h>
 #if defined(__has_include)
 #  if __has_include(<sys/syscall.h>)
 #    include <sys/syscall.h>
 #  endif
+#  if __has_include(<unistd.h>)
+#    include <unistd.h>
+#  endif
 #endif
 
 #include <stdlib.h>