fix: declare opendmarc_policy_fetch_fo() in dmarc.h.in (#407)

The function was implemented and documented but never declared in the
public header, causing a build failure on Clang 15+ where implicit
function declarations are an error rather than a warning.

Author: thegushi

libopendmarc/dmarc.h.in

@@ -142,6 +142,7 @@ OPENDMARC_STATUS_T opendmarc_policy_fetch_adkim(DMARC_POLICY_T *pctx, int *adkim
 OPENDMARC_STATUS_T opendmarc_policy_fetch_aspf(DMARC_POLICY_T *pctx, int *aspf);
 OPENDMARC_STATUS_T opendmarc_policy_fetch_p(DMARC_POLICY_T *pctx, int *p);
 OPENDMARC_STATUS_T opendmarc_policy_fetch_sp(DMARC_POLICY_T *pctx, int *sp);
+OPENDMARC_STATUS_T opendmarc_policy_fetch_fo(DMARC_POLICY_T *pctx, int *fo);
 u_char **	   opendmarc_policy_fetch_rua(DMARC_POLICY_T *pctx, u_char *list_buf, size_t size_of_buf, int constant);
 u_char **	   opendmarc_policy_fetch_ruf(DMARC_POLICY_T *pctx, u_char *list_buf, size_t size_of_buf, int constant);
 OPENDMARC_STATUS_T opendmarc_policy_fetch_utilized_domain(DMARC_POLICY_T *pctx, u_char *buf, size_t buflen);