Expose frameSrcSize to experimental API

lib/zstd.h

@@ -396,6 +396,17 @@ typedef void* (*ZSTD_allocFunction) (void* opaque, size_t size);
 typedef void  (*ZSTD_freeFunction) (void* opaque, void* address);
 typedef struct { ZSTD_allocFunction customAlloc; ZSTD_freeFunction customFree; void* opaque; } ZSTD_customMem;
 
+/***************************************
+*  Compressed size functions
+***************************************/
+
+/*! ZSTD_getFrameCompressedSize() :
+ *  `src` should point to the start of a ZSTD encoded frame
+ *  `srcSize` must be at least as large as the frame
+ *  @return : the compressed size of the frame pointed to by `src`, suitable to pass to
+ *      `ZSTD_decompress` or similar, or an error code if given invalid input. */
+ZSTDLIB_API size_t ZSTD_getFrameCompressedSize(const void* src, size_t srcSize);
+
 /***************************************
 *  Decompressed size functions
 ***************************************/