Fix GH-13932: Attempt to fix mbstring on windows build (msvc).

Author: devnexen

ext/mbstring/mbstring.c

@@ -5883,8 +5883,8 @@ static zend_string* mb_mime_header_encode(zend_string *input, const mbfl_encodin
 	unsigned int state = 0;
 	/* wchar_buf should be big enough that when it is full, we definitely have enough
 	 * wchars to fill an entire line of output */
-	const size_t wchar_buf_len = 90;
-	uint32_t wchar_buf[wchar_buf_len];
+	size_t wchar_buf_len = 90;
+	uint32_t wchar_buf[90];
 	uint32_t *p, *e;
 	/* What part of wchar_buf is filled with still-unprocessed data which should not
 	 * be overwritten? */