gnu: asciidoc: Restore custom 'bootstrap' phase.

Fix a regression introduced by commit
189be331acfda1c242a9c85fca8d2a0356742f48.

* gnu/packages/documentation.scm (asciidoc)[arguments]: Restore the custom
'bootstrap' phase that present prior to commit 189be33.

gnu/packages/documentation.scm

@@ -59,6 +59,9 @@
      `(#:tests? #f                     ; no 'check' target
        #:phases
        (modify-phases %standard-phases
+         (replace 'bootstrap
+           (lambda _
+             (invoke "autoconf")))
          ;; Some XML-related binaries are required for asciidoc's proper usage.
          ;; Without these, asciidoc fails when parsing XML documents, either
          ;; reporting a missing "xmllint" binary or, when passed the