Fix zstd-dll build missing dependencies (#3496)

* Fixes zstd-dll build (https://github.com/facebook/zstd/issues/3492):
- Adds pool.o and threading.o dependency to the zstd-dll target
- Moves custom allocation functions into header to avoid needing to add dependency on common.o
- Adds test target for zstd-dll
- Adds github workflow that buildis zstd-dll

lib/common/pool.c

@@ -10,9 +10,9 @@
 
 
 /* ======   Dependencies   ======= */
+#include "../common/allocations.h"  /* ZSTD_customCalloc, ZSTD_customFree */
 #include "zstd_deps.h" /* size_t */
 #include "debug.h"     /* assert */
-#include "zstd_internal.h"  /* ZSTD_customCalloc, ZSTD_customFree */
 #include "pool.h"
 
 /* ======   Compiler specifics   ====== */