lib/legacy: warning: this statement may fall through

lib/legacy: warning: this statement may fall through

The following warning appears during build at sevaral places.

../lib/legacy/zstd_v04.c:819:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
             case 7: bitD->bitContainer += (size_t)(((const BYTE*)(bitD->start))[6]) << (sizeof(size_t)*8 - 16);

../lib/legacy/zstd_v05.c:821:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
             case 7: bitD->bitContainer += (size_t)(((const BYTE*)(bitD->start))[6]) << (sizeof(size_t)*8 - 16);

../lib/legacy/zstd_v06.c:913:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
             case 7: bitD->bitContainer += (size_t)(((const BYTE*)(srcBuffer))[6]) << (sizeof(bitD->bitContainer)*8 - 16);

../lib/legacy/zstd_v07.c:583:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
             case 7: bitD->bitContainer += (size_t)(((const BYTE*)(srcBuffer))[6]) <<
             (sizeof(bitD->bitContainer)*8 - 16);

Signed-off-by: Jos Collin <jcollin@redhat.com>