lib: posix: device_io: Add standard file descriptor sizes

Make sure we always allocate space for stdin, stdout and stderr file
descriptors if Posix device io option is enabled.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

lib/posix/options/Kconfig.device_io

@@ -43,6 +43,11 @@ config POSIX_DEVICE_IO_ALIAS_WRITE
 	help
 	  When selected via Kconfig, Zephyr will provide an alias for write() as _write().
 
+# Allocate fd for stdin, stdout and stderr
+config ZVFS_OPEN_ADD_SIZE_POSIX_DEVICE_IO
+	int
+	default 3
+
 endif # POSIX_DEVICE_IO
 
 config POSIX_OPEN_MAX