Use docopt-cpp from Buildroot upstream

Change-Id: If76dd0d71fe38b530493e3252293e6522520dbd0
diff --git a/ci/build.sh b/ci/build.sh
index 14e73bb..14b7055 100755
--- a/ci/build.sh
+++ b/ci/build.sh
@@ -76,7 +76,7 @@
     for PROJECT in \
             libyang sysrepo libnetconf2 netopeer2 \
             libyang-cpp sysrepo-cpp \
-            docopt-cpp replxx cppcodec sdbus-cpp \
+            replxx cppcodec sdbus-cpp \
             ; do
         rm -rf build/{,host-}${PROJECT}-custom/ per-package/{,host-}${PROJECT}/
     done