wrlib: Bump soname version to 6.

wrlib: Bump soname version to 6.

Usually, a soname bump is reserved for when an interface is added, changed,
or removed.  This has not happened to wrlib.  However, I think there is a
special circumstance that warrants the bump.

In particular, there are two versions of wrlib version 5 in the wild:
* The one that shipped with wmaker 0.95.6, which uses symbol version
  LIBWRASTER3.
* The one that shipped with wmaker 0.95.7, which uses symbol version
  LIBWRASTER5.
This was reported in Debian bug #811304 [1].  Binaries which were built
against the older wrlib won't run after upgrading to the newer one, even
though they are supposedly using the same soname version.

(We get around this currently in Debian by patching the old LIBWRASTER3
symbol version back in.)

I propose we bump to version 6 and put this mess behind us.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=811304