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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/blackfin/Makefile

···
 ifneq ($(CONFIG_BLACKFIN),)
-ifneq ($(CONFIG_BFIN_GPTIMERS,)
+ifneq ($(CONFIG_BFIN_GPTIMERS),)
 obj-m := gptimers-example.o
 endif
 endif