build: Also update Dockerfile.buildx for new package dependency

Dockerfile.buildx

@@ -5,7 +5,7 @@ EXPOSE 8384 22000/tcp 22000/udp 21027/udp
 
 VOLUME ["/var/syncthing"]
 
-RUN apk add --no-cache ca-certificates su-exec tzdata
+RUN apk add --no-cache ca-certificates su-exec tzdata libcap
 
 COPY ./syncthing-linux-$TARGETARCH /bin/syncthing
 COPY ./script/docker-entrypoint.sh /bin/entrypoint.sh