ffind and sleuthkit conflict

Closes Homebrew/homebrew#15593.

Formula/ffind.rb

@@ -5,6 +5,9 @@ class Ffind < Formula
   url 'https://github.com/sjl/friendly-find/tarball/v0.3.1'
   sha1 '891207067c0d8715b48e91a52ff815aa73d7e139'
 
+  conflicts_with 'sleuthkit',
+    :because => "both install a 'ffind' executable."
+
   def install
     bin.install "ffind"
   end

Formula/sleuthkit.rb

@@ -16,6 +16,9 @@ class Sleuthkit < Formula
   depends_on 'afflib' => :optional
   depends_on 'libewf' => :optional
 
+  conflicts_with 'ffind',
+    :because => "both install a 'ffind' executable."
+
   def install
     system "./bootstrap" if build.head?
     system "./configure", "--disable-debug", "--disable-dependency-tracking",