Fix decompression of files with multiple frames.

[DiThi]

Decompressing a file with more than one frame fails with the message "Destination buffer is too small". This is because ZSTD_getFrameContentSizeonly gives the size of the first frame.

The solution is to use ZSTD_findDecompressedSize. However, it is in a section of the header that is disabled when not using ZSTD_STATIC_LINKING_ONLY. To avoid having to define it I just added cdecl, so nim makes the declaration instead of just using it.