Documentation: blackfin: Makefile: Typo building issue

Miss a ')' for ifneq in Makefile, the related building error:

  Documentation/blackfin/Makefile:2: *** invalid syntax in conditional.  Stop.
  make[1]: *** [Documentation/blackfin] Error 2

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>