fcntl: make mutate_flag optional for ioctl w/ read-only buffer (#3882)

Fixes #3881

Author: julian-klode

stdlib/3/fcntl.pyi

@@ -111,7 +111,7 @@ def ioctl(__fd: FileDescriptorLike,
 def ioctl(__fd: FileDescriptorLike,
           __request: int,
           __arg: _ReadOnlyBuffer,
-          __mutate_flag: bool) -> bytes: ...
+          __mutate_flag: bool = ...) -> bytes: ...
 def flock(__fd: FileDescriptorLike, __operation: int) -> None: ...
 def lockf(__fd: FileDescriptorLike,
           __cmd: int,