kconfig: qconf: fix incomplete type 'struct gstr' warning

"make HOSTCXX=clang++ xconfig" reports the following:

  HOSTCXX scripts/kconfig/qconf.o
In file included from scripts/kconfig/qconf.cc:23:
In file included from scripts/kconfig/lkc.h:15:
scripts/kconfig/lkc_proto.h:26:13: warning: 'get_relations_str' has C-linkage specified, but returns incomplete type 'struct gstr' which could be incompatible with C [-Wreturn-type-c-linkage]
struct gstr get_relations_str(struct symbol **sym_arr, struct list_head *head);
            ^

Currently, get_relations_str() is declared before the struct gstr
definition.

Move all declarations of menu.c functions below.

BTW, some are declared in lkc.h and some in lkc_proto.h, but the
difference is unclear. I guess some refactoring is needed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Boris Kolpackov <boris@codesynthesis.com>