A build in libasn1 always creates new header files

[Bernd Ernesti <netbsd%lists.veego.de@localhost>]

Hi,

it seems that the new src/lib/libasn1 build always creates new
header files each time a make is executed in src/lib/libasn1.

This is now casing a rebuild of everthing in src/lib/libasn1
if you just do a make, despite no other files changed in the
source code.

The old Makefile had code to generate a .hx file and compare
that with the real .h file, but this was removed in rev. 1.29

Bernd