appveyor.dpkg: Install libfmt-dev.

NOTE: Ubuntu 16.04 doesn't have libfmt-dev, so rom-properties will no
longer build on Ubuntu 16.04 until an internal copy is added.

We'll probably use a header-only build for Ubuntu 16.04, and a DLL
build for Windows.

appveyor-dpkg.sh

@@ -14,6 +14,7 @@ apt-get -y install \
 	libtinyxml2-dev \
 	gettext \
 	libseccomp-dev \
+	libfmt-dev \
 	\
 	libzstd-dev \
 	liblz4-dev \