libprivatezstd: Restrict symbol visibility similar to upstream

We have to relax three symbols to dynamically link zstd(1), but the rest
are the same.

lib/libzstd/Makefile

@@ -36,7 +36,7 @@ SRCS=	entropy_common.c \
 WARNS?=	2
 INCS=	zstd.h
 CFLAGS+=	-I${ZSTDDIR}/lib -I${ZSTDDIR}/lib/common -DXXH_NAMESPACE=ZSTD_ \
-		-DZSTD_MULTITHREAD=1
+		-DZSTD_MULTITHREAD=1 -fvisibility=hidden
 LIBADD=	pthread
 
 PRIVATELIB=	yes

sys/contrib/zstd/lib/common/xxhash.h

@@ -141,6 +141,9 @@ typedef unsigned int       XXH32_hash_t;
 typedef unsigned long long XXH64_hash_t;
 
 XXH_PUBLIC_API XXH32_hash_t XXH32 (const void* input, size_t length, unsigned int seed);
+/* Begin FreeBSD - This symbol is needed by dll-linked CLI zstd(1). */
+__attribute__((visibility ("default")))
+/* End FreeBSD */
 XXH_PUBLIC_API XXH64_hash_t XXH64 (const void* input, size_t length, unsigned long long seed);
 
 /*!

sys/contrib/zstd/lib/dictBuilder/zdict.c

@@ -968,6 +968,9 @@ static size_t ZDICT_addEntropyTablesFromBuffer_advanced(
 }
 
 /* Hidden declaration for dbio.c */
+/* Begin FreeBSD - This symbol is needed by dll-linked CLI zstd(1). */
+ZSTDLIB_API
+/* End FreeBSD */
 size_t ZDICT_trainFromBuffer_unsafe_legacy(
                             void* dictBuffer, size_t maxDictSize,
                             const void* samplesBuffer, const size_t* samplesSizes, unsigned nbSamples,