kernel-doc: bugfix - empty line in Example section

If you have a section named "Example" that contains an empty line,
attempting to generate htmldocs give you the error:

/path/Documentation/DocBook/kernel-api.xml:3455: parser error : Opening and ending tag mismatch: programlisting line 3449 and para
   </para><para>
          ^
/path/Documentation/DocBook/kernel-api.xml:3473: parser error : Opening and ending tag mismatch: para line 3467 and programlisting
</programlisting></informalexample>
                 ^
/path/Documentation/DocBook/kernel-api.xml:3678: parser error : Opening and ending tag mismatch: programlisting line 3672 and para
   </para><para>
          ^
/path/Documentation/DocBook/kernel-api.xml:3701: parser error : Opening and ending tag mismatch: para line 3690 and programlisting
</programlisting></informalexample>
                 ^
unable to parse
/path/Documentation/DocBook/kernel-api.xml

Essentially, the script attempts to close a <programlisting> with a
closing tag for a <para> block.  This patch corrects the problem by
simply not outputting anything extra when we're dumping pre-formatted
text, since the empty line will be rendered correctly anyway.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>