jmtpfs: add platform

pkgs/tools/filesystems/jmtpfs/default.nix

···
     description = "A FUSE filesystem for MTP devices like Android phones";
     homepage = https://github.com/JasonFerrara/jmtpfs;
     license = licenses.gpl3;
+    platforms = platforms.linux;
     maintainers = [ maintainers.coconnor ];
   };
 }