build: fix --enable-rxgk help format

build: fix --enable-rxgk help format

Move the dnl macros out of the AC_ARG_ENABLE to fix the
formatting of the --enable-rxgk help string.

Before this commit:

  $ ./configure --help | grep -C2 rxgk

    --enable-kauth          install the deprecated kauth server, pam modules,
                            and utilities (defaults to disabled)
        --enable-rxgk           Include experimental support for the RXGK security
                            class (defaults to disabled)
    --disable-strip-binaries

After this commit:

  $ ./configure --help | grep -C2 rxgk

    --enable-kauth          install the deprecated kauth server, pam modules,
                            and utilities (defaults to disabled)
    --enable-rxgk           Include experimental support for the RXGK security
                            class (defaults to disabled)
    --disable-strip-binaries

Change-Id: Iaf6695643f11c7b636e3fba33ee7161e21df23a6
Reviewed-on: https://gerrit.openafs.org/13722
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>