Fixed missing FSctl symbol

This commit fixes the following error encountered by the
Continuous Integration system and discovered and
reported by Eric:

Config Falling back to using libXrdCtaOfs.so
Plugin /lib64/libXrdCtaOfs.so: undefined symbol: _ZN3cta13xroot_plugins16XrdCtaFilesystem5FSctlEiR11XrdSfsFSctlR13XrdOucErrInfoPK12XrdSecEntity fslib libXrdCtaOfs.so
Config Unable to load fslib plugin libXrdCtaOfs.so
170804 02:37:03 558 XrootdConfig: Unable to create file system object via libXrdCtaOfs.so
170804 02:37:03 558 XrootdConfig: Unable to load file system.
------ xrootd protocol initialization failed.

The origin of the problem is the following commit that
completed removed the implemention of the
XrdCtaFilesystem::FSctl() method:

commit ea8d5241
Author: Michael Davis <michael.davis@cern.ch>
Date:   Thu Aug 3 10:54:49 2017 +0200

    [XrdSsi] Updates eos_messages.proto and deletes Opaque Query

    Update the protobuf file to the version required by EOS-CTA SSI
    interface and delete all source code that depends on the previous
    version (i.e. all the opaque query code).

This removal should have been a replacement as opposed
to a hard delete.  The XrdCtaFilesystem::FSctl() method
should have been re-implemented as follows:

int XrdCtaFilesystem::FSctl(const int cmd, XrdSfsFSctl &args, XrdOucErrInfo &eI
  (void)cmd; (void)args; (void)eInfo; (void)client;
  eInfo.setErrInfo(ENOTSUP, "Not supported.");
  return SFS_ERROR;
}

xroot_plugins/XrdCtaFilesystem.cpp

@@ -89,6 +89,15 @@ XrdSfsDirectory * XrdCtaFilesystem::newDir(char *user, int MonID)
   return new cta::xrootPlugins::XrdCtaDir(m_catalogue.get(), m_log.get(), user, MonID);;
 }
 
+//------------------------------------------------------------------------------
+// FSctl
+//------------------------------------------------------------------------------
+int XrdCtaFilesystem::FSctl(const int cmd, XrdSfsFSctl &args, XrdOucErrInfo &eInfo, const XrdSecEntity *client) {
+  (void)cmd; (void)args; (void)eInfo; (void)client;
+  eInfo.setErrInfo(ENOTSUP, "Not supported.");
+  return SFS_ERROR;
+}
+
 //------------------------------------------------------------------------------
 // fsctl
 //------------------------------------------------------------------------------