set java_websocket source as UTF-8

to avoid

    [javac] core/workdir/UnpackedTarball/java_websocket/src/main/java/org/java_websocket/util/Charsetfunctions.java:88: error: unmappable character (0xC3) for encoding US-ASCII
    [javac]    * Implementation of the "Flexible and Economical UTF-8 Decoder" algorithm by Bj??rn H??hrmann

with LANG=C

Change-Id: Idc7296101f18ef4d0cbedb232834b1cc674444c9
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/156552
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

external/java_websocket/patches/ant-build.patch

@@ -7,7 +7,7 @@
 +    <target name="compile">
 +        <mkdir dir="build/classes" />
 +        <javac includeantruntime="false" srcdir="src/main/java"
-+               destdir="build/classes">
++               destdir="build/classes" encoding="UTF-8">
 +        </javac>
 +    </target>
 +