textproc/scdoc: unbreak build with clang 15

In file included from src/string.c:3:
include/str.h:10:23: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
struct str *str_create();

^
 void

src/string.c:15:23: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
struct str *str_create() {

^
 void

PR: 268277
Reported by: antoine (via bug 265425 exp-run)