gh-134768: Fix definition of mt_continue_should_break()

In 121ed71, mt_continue_should_break
was changed to be guarded by `Py_DEBUG`, but it's used in `compress_mt_continue_lock_held`
with just `assert`, so it needs to be available when `NDEBUG` is undefined
too.

`Py_DEBUG` implies `NDEBUG` is undefined, so we can check just that.

Fixes: 121ed71

Author: thesamesam

Modules/_zstd/compressor.c

@@ -489,7 +489,7 @@ compress_lock_held(ZstdCompressor *self, Py_buffer *data,
     return NULL;
 }
 
-#ifdef Py_DEBUG
+#ifndef NDEBUG
 static inline int
 mt_continue_should_break(ZSTD_inBuffer *in, ZSTD_outBuffer *out)
 {