docs: Add libssl-dev to prerequisites

libssl-dev is needed to compile the ARM-TF fiptool when compiling for Juno.

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

README.md

@@ -117,9 +117,9 @@ $ sudo apt-get install android-tools-adb android-tools-fastboot autoconf \
 	automake bc bison build-essential cscope curl flex ftp-upload gdisk \
 	libattr1-dev libc6:i386 libcap-dev libfdt-dev libftdi-dev \
 	libglib2.0-dev libhidapi-dev libncurses5-dev libpixman-1-dev \
-	libstdc++6:i386 libtool libz1:i386 make mtools netcat python-crypto \
-	python-serial python-wand unzip uuid-dev xdg-utils xterm xz-utils \
-	zlib1g-dev
+	libssl-dev libstdc++6:i386 libtool libz1:i386 make mtools netcat \
+	python-crypto python-serial python-wand unzip uuid-dev xdg-utils \
+	xterm xz-utils zlib1g-dev
 ```
 
 ## 7.2 Install Android repo