cmake win32 change helper includes to HDR_PUBLIC so they install

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2d5f962..e65eb83 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -270,7 +270,7 @@
 if (WIN32)
 	set(WIN32_HELPERS_PATH win32port/win32helpers)
 
-	list(APPEND HDR_PRIVATE
+	list(APPEND HDR_PUBLIC
 		${WIN32_HELPERS_PATH}/websock-w32.h
 		${WIN32_HELPERS_PATH}/gettimeofday.h
 		)