unix: link librt into _posixshmem extension

Without this we have missing symbols. I think this regressed as part of
upgrading the Debian version in the build environment.

Author: indygreg

cpython-unix/static-modules.3.8.linux64

@@ -21,7 +21,7 @@ _lzma _lzmamodule.c -I/tools/deps/include -L/tools/deps/lib -llzma
 # dependency.
 _multiprocessing _multiprocessing/multiprocessing.c _multiprocessing/semaphore.c
 _opcode _opcode.c
-_posixshmem _multiprocessing/posixshmem.c -IModules/_multiprocessing
+_posixshmem _multiprocessing/posixshmem.c -IModules/_multiprocessing -lrt
 _queue _queuemodule.c
 _sqlite3 _sqlite/cache.c _sqlite/connection.c _sqlite/cursor.c _sqlite/microprotocols.c _sqlite/module.c _sqlite/prepare_protocol.c _sqlite/row.c _sqlite/statement.c _sqlite/util.c -I/tools/deps/include -IModules/_sqlite -DMODULE_NAME=\"sqlite3\" -DSQLITE_OMIT_LOAD_EXTENSION=1 -L/tools/deps/lib -lsqlite3
 _ssl _ssl.c -I/tools/deps/include -lssl -lcrypto