zstd: Fix non-FreeBSD CI libzstd build

Fix non-FreeBSD CI build after v1.4.8.  This definition was only used in
zstd(1), which isn't part of non-FreeBSD CI (I guess).  The ifdef was
added in v1.4.5 import.

Upstream does not currently support shared-linked zstd(1), but I have
proposed facebook/zstd#2450 .  If that is
adopted, we can add -DZSTD_PROGRAMS_LINK_SHARED to our libzstd build and
drop some diffs.

Reported by:	uqs

sys/contrib/zstd/lib/compress/zstd_compress_internal.h

@@ -1198,8 +1198,9 @@ size_t ZSTD_referenceExternalSequences(ZSTD_CCtx* cctx, rawSeq* seq, size_t nbSe
 
 /** ZSTD_cycleLog() :
  *  condition for correct operation : hashLog > 1 */
-#ifdef	__FreeBSD__	/* This symbol is needed by dll-linked CLI zstd(1). */
-ZSTDLIB_API U32 ZSTD_cycleLog(U32 hashLog, ZSTD_strategy strat);
-#endif
+/* Begin FreeBSD - This symbol is needed by dll-linked CLI zstd(1). */
+ZSTDLIB_API
+/* End FreeBSD */
+U32 ZSTD_cycleLog(U32 hashLog, ZSTD_strategy strat);
 
 #endif /* ZSTD_COMPRESS_H */