readme: add build-essential to Ubuntu build dependencies

otherwise only gcc will be available on a fresh Ubuntu
install (tested with 20.04), but g++ is missing.
This installs both.

README.md

@@ -119,7 +119,7 @@ Build Instructions
 
 For example, on Ubuntu, one can install the mandatory build dependencies with:
 ```
-sudo apt-get install cmake libfftw3-dev libmbedtls-dev libboost-program-options-dev libconfig++-dev libsctp-dev
+sudo apt-get install build-essential cmake libfftw3-dev libmbedtls-dev libboost-program-options-dev libconfig++-dev libsctp-dev
 ```
 or on Fedora:
 ```