singular 4.4.1: add flint as a dependency to Singular

Singular has an optional interface for flint, which is not built in the
current formula as it's missing. We add the dependency to let
it build it. Basically, it makes the executable a bit bigger and
links it (onb my machine, currently) to
 /usr/local/opt/flint/lib/libflint.20.0.dylib

Author: dimpase

Formula/s/singular.rb

@@ -52,6 +52,7 @@ class Singular < Formula
     depends_on "libtool" => :build
   end
 
+  depends_on "flint"
   depends_on "gmp"
   depends_on "mpfr"
   depends_on "ntl"