Allow some packages to build on darwin as well

(cherry picked from commit 80e730c9a41ee327308144ffcde47f03a148e7c2)

pkgs/development/libraries/rabbitmq-c/default.nix

···
     description = "RabbitMQ C AMQP client library";
     homepage = https://github.com/alanxz/rabbitmq-c;
     license = licenses.mit;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ wkennington ];
   };
 }

pkgs/servers/mail/dovecot/plugins/pigeonhole/default.nix

···
     description = "A sieve plugin for the Dovecot IMAP server";
     license = licenses.lgpl21;
     maintainers = [ maintainers.rickynils ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }

pkgs/tools/misc/plantuml/default.nix

···
     # "java -jar plantuml.jar -license" says GPLv3 or later
     license = licenses.gpl3Plus;
     maintainers = [ maintainers.bjornfor ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }

pkgs/tools/security/paperkey/default.nix

···
     '';
     homepage = http://www.jabberwocky.com/software/paperkey/;
     license = licenses.gpl2;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ skeidel ];
   };
 }

pkgs/tools/security/srm/default.nix

···
     homepage = http://srm.sourceforge.net;
     license = licenses.mit;
     maintainers = with maintainers; [ edwtjo ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 
-}
+}