FAB-2684 Doc update for missing header file

The inclusion of the miekg/pkcs11 package introduced
an external dependency on the libtdl.h header file during
a build of fabric-ca. This will be addressed by updating
the instructions to indicate the explicit need (an instructions
for installing) the libtool and libltdl-dev packages.

Change-Id: I68be86542999b8c1c486baf17178f5b2bfc61ff3
Signed-off-by: rennman <eabailey@us.ibm.com>

docs/source/Setup/ca-setup.rst

@@ -27,6 +27,17 @@ Prerequisites
 
 -  Go 1.7+ installation or later
 -  **GOPATH** environment variable is set correctly
+- libtool and libtdhl-dev packages are installed
+
+The following installs the libtool dependencies.
+
+::
+
+   # sudo apt install libtool libltdl-dev
+
+For more information on libtool, see https://www.gnu.org/software/libtool.
+
+For more information on libtdhr-dev, see https://www.gnu.org/software/libtool/manual/html_node/Using-libltdl.html.
 
 Install
 ~~~~~~~