docs: add libfmt as a required dependancy

doc/build-osx.md

@@ -19,7 +19,7 @@ Then install [Homebrew](https://brew.sh).
 
 ## Dependencies
 ```shell
-brew install automake libtool boost miniupnpc pkg-config python qt libevent qrencode
+brew install automake libtool boost miniupnpc pkg-config python qt libevent qrencode fmt
 ```
 
 If you run into issues, check [Homebrew's troubleshooting page](https://docs.brew.sh/Troubleshooting).

doc/build-unix.md

@@ -81,7 +81,7 @@ Build requirements:
 
 Now, you can either build from self-compiled [depends](/depends/README.md) or install the required dependencies:
 
-    sudo apt-get install libevent-dev libboost-system-dev libboost-filesystem-dev libboost-test-dev libboost-thread-dev
+    sudo apt-get install libevent-dev libboost-system-dev libboost-filesystem-dev libboost-test-dev libboost-thread-dev libfmt-dev
 
 BerkeleyDB is required for the wallet.
 
@@ -131,7 +131,7 @@ built by default.
 
 Build requirements:
 
-    sudo dnf install gcc-c++ libtool make autoconf automake libevent-devel boost-devel libdb4-devel libdb4-cxx-devel python3
+    sudo dnf install gcc-c++ libtool make autoconf automake libevent-devel boost-devel libdb4-devel libdb4-cxx-devel python3 fmt
 
 Optional (see `--with-miniupnpc` and `--enable-upnp-default`):