linkage_checker: fix generic pathname calls.

Library/Homebrew/extend/pathname.rb

@@ -472,9 +472,17 @@ class Pathname
     }
   end
 
+  def binary_executable?
+    false
+  end
+
   def mach_o_bundle?
     false
   end
+
+  def dylib?
+    false
+  end
 end
 
 require "extend/os/pathname"

Library/Homebrew/linkage_checker.rb

@@ -66,7 +66,7 @@ class LinkageChecker
     checked_dylibs = Set.new
     @keg.find do |file|
       next if file.symlink? || file.directory?
-      next unless file.dylib? || file.binary_executable? || file.mach_o_bundle?
+      next if !file.dylib? && !file.binary_executable? && !file.mach_o_bundle?
 
       # weakly loaded dylibs may not actually exist on disk, so skip them
       # when checking for broken linkage