pinctrl: generic: Fix compilation error

The function definition of pinconf_generic_dump_config is defined
under CONFIG_DEBUG_FS macro. Define the declaration too under this macro.

Without this patch we get the following build error:
drivers/built-in.o: In function `pcs_pinconf_config_dbg_show':
drivers/pinctrl/pinctrl-single.c:726: undefined reference to
`pinconf_generic_dump_config'

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>