added ZSTD_COMPRESSBOUND() as a macro

ZSTD_compressBound() works fine, but is only useful for dynamic allocation.
For static allocation, only a macro can provide the amount during compilation time.

lib/compress/zstd_compress.c

@@ -38,9 +38,7 @@
 *  Helper functions
 ***************************************/
 size_t ZSTD_compressBound(size_t srcSize) {
-    size_t const lowLimit = 256 KB;
-    size_t const margin = (srcSize < lowLimit) ? (lowLimit-srcSize) >> 12 : 0;  /* from 64 to 0 */
-    return srcSize + (srcSize >> 8) + margin;
+    return ZSTD_COMPRESSBOUND(srcSize);
 }