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'