User guide: Add dependency on libssl-dev for cert_create tool

The 'libssl-dev' package must be installed on the host to build the
certificate generation tool. This patch adds it to the list of
required tools in the User Guide.

Change-Id: I018381fb14b7c2d2bd6f2b7929aaad0571f7eb2e

docs/user-guide.md

@@ -59,6 +59,9 @@ The following tools are required to use the ARM Trusted Firmware:
         wget http://releases.linaro.org/14.07/components/toolchain/binaries/gcc-linaro-aarch64-none-elf-4.9-2014.07_linux.tar.xz
         tar -xf gcc-linaro-aarch64-none-elf-4.9-2014.07_linux.tar.xz
 
+*   `libssl-dev` package to build the certificate generation tool when support
+    for Trusted Board Boot is needed.
+
 *   (Optional) For debugging, ARM [Development Studio 5 (DS-5)][DS-5] v5.20.