Fix code example by adding a missing import.
Fixes #1557890.
Backported from trunk.
diff --git a/Doc/lib/libctypes.tex b/Doc/lib/libctypes.tex
index 2bae25b..8474a44 100755
--- a/Doc/lib/libctypes.tex
+++ b/Doc/lib/libctypes.tex
@@ -1848,7 +1848,7 @@
Here is the wrapping with \code{ctypes}:
\begin{quote}
-\begin{verbatim}>>> from ctypes import POINTER, WINFUNCTYPE, windll
+\begin{verbatim}>>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
>>> from ctypes.wintypes import BOOL, HWND, RECT
>>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
>>> paramflags = (1, "hwnd"), (2, "lprect")