Add libleveldb-dev to build dependencies

Fixes "leveldb/db.h: No such file or directory" error.

INSTALL.md

@@ -25,7 +25,7 @@ sudo swapon /swapfile
 # install build dependencies
 sudo apt-get install build-essential pkg-config libc6-dev m4 g++-multilib \
       autoconf libtool ncurses-dev unzip git zlib1g-dev wget \
-      bsdmainutils automake curl unzip nano libsodium-dev cmake
+      bsdmainutils automake curl unzip nano libsodium-dev libleveldb-dev cmake
 # clone git repo
 git clone https://git.hush.is/hush/hush3
 cd hush3