remove redundant redeclarations

All three functions gave similar warning, here is a sample who the
message text looked.

src/libabc.c:236:24: warning: redundant redeclaration of 'abc_list_entry_get_next' [-Wredundant-decls]
 struct abc_list_entry *abc_list_entry_get_next(struct abc_list_entry *list_entry);
                        ^
In file included from src/libabc.c:30:0:
./src/abc/libabc.h:55:24: note: previous declaration of 'abc_list_entry_get_next' was here
 struct abc_list_entry *abc_list_entry_get_next(struct abc_list_entry *list_entry);
1 file changed
tree: 7f57d9ca4638b6a91381dfe9a0c75dd39a526d10
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