Fix building of folly/compression with modules

Summary: Move the definition of the `ZSTD_STATIC_LINKING_ONLY` and `ZDICT_STATIC_LINKING_ONLY` macros from code to the target to fix the modular build. It is needed because these macros affect the public API of `zstd.h`.

Reviewed By: terrelln

Differential Revision: D9189720

fbshipit-source-id: efacc1456803900fd65159a43cf1fba2daf15673

folly/compression/Zstd.cpp

@@ -20,7 +20,6 @@
 #include <stdexcept>
 #include <string>
 
-#define ZSTD_STATIC_LINKING_ONLY
 #include <zstd.h>
 
 #include <folly/Conv.h>

folly/compression/Zstd.h

@@ -24,7 +24,9 @@
 
 #if FOLLY_HAVE_LIBZSTD
 
+#ifndef ZSTD_STATIC_LINKING_ONLY
 #define ZSTD_STATIC_LINKING_ONLY
+#endif
 #include <zstd.h>
 
 namespace folly {