kconfig: Remove gperf 3.1+ incompatible kconf_conf_id forward declaration
Forward declaration was unused. gperf 3.1+ changed parameter type from
'unsigned int' to 'size_t'.
Ref github #200 https://github.com/espressif/esp-idf/issues/200#issuecomment-
279377753
Fixes #315 https://github.com/espressif/esp-idf/issues/315
Fixes #251 https://github.com/espressif/esp-idf/issues/251
See merge request !498