Emscripten: fixed build script

os/emscripten/emscripten-build.sh

@@ -57,6 +57,7 @@ mkdir -p baseset
 }
 
 [ -e Makefile ] || emcmake cmake .. -DHOST_BINARY_DIR=$(pwd)/build-host -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DOPTION_USE_ASSERTS=OFF || exit 1
+rm -f openttd.html
 emmake make -j8 VERBOSE=1 || exit 1
 
 cp -f *.html *.js *.data *.wasm ../media/openttd.256.png ../os/emscripten/openttd.webapp /var/www/html/