libstdc++: Fix std::random_device for avr

This fixes a build failure that affects avr, but could affect other
targets in theory. The _M_fini function should not try to use ::open or
::fopen if _GLIBCXX_USE_DEV_RANDOM is not defined, because no file can
ever have been opened.

libstdc++-v3/ChangeLog:

	* src/c++11/random.cc (random_device::_M_fini): Do not try to
	close the file handle if the target doesn't support the
	/dev/random and /dev/urandom files.

libstdc++-v3/src/c++11/random.cc

@@ -548,6 +548,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
       }
 #endif
 
+#ifdef _GLIBCXX_USE_DEV_RANDOM
 #ifdef USE_POSIX_FILE_IO
     ::close(_M_fd);
     _M_fd = -1;
@@ -555,6 +556,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
     std::fclose(static_cast<FILE*>(_M_file));
 #endif
     _M_file = nullptr;
+#endif
   }
 
   random_device::result_type