component/proftpd: add libzstd in rpath for mod_auth_web

mod_auth_web uses libcurl. Since 83f5929e (component/curl: version up 7.76.0,
including security fixes., 2021-04-07), our curl build depends on zstd.
Apply similar fix that 2f4d8cf8 (component/apache-php: fix missing libzstd, 2021-04-09),
ie. add zstd to rpath

component/proftpd/buildout.cfg

@@ -12,6 +12,7 @@ extends =
   ../libtool/buildout.cfg
   ../git/buildout.cfg
   ../patch/buildout.cfg
+  ../zstd/buildout.cfg
 
 # proftpd server
 [proftpd-environment]
@@ -61,7 +62,7 @@ command =
   LIBTOOL=${libtool:location}/bin/libtool \
   ${proftpd-output:prxs} -c -i -d \
   -I ${curl:location}/include \
-  -L '${curl:location}/lib/ -R ${openssl:location}/lib -R ${zlib:location}/lib' \
+  -L '${curl:location}/lib/ -R ${openssl:location}/lib -R ${zlib:location}/lib -R ${zstd:location}/lib' \
   -l curl \
   ${proftpd-mod_auth_web-repository:location}/mod_auth_web.c
 location=${proftpd:location}/libexec/mod_auth_web.so