samples: civetweb: websocket_server: Fix build issue

The sample uses generate_inc_file_for_target for some "webpages" that
get included.  However, the directory that the generated files are
put into: ${ZEPHYR_BINARY_DIR}/include/generated/web_page isn't
created and thus the build fails.

Add a simple 'file(MAKE_DIRECTORY ..)' to create the dir.

Fixes #34345

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
1 file changed