Move #includes out of extern "C" blocks

Do some include shuffling for `**.h` files within lib, programs, tests, and zlibWrapper.
`lib/legacy` and `lib/deprecated` are untouched.
`#include`s within `extern "C"` blocks in .cpp files are untouched.

todo: shuffling for `xxhash.h`

programs/fileio_asyncio.h

@@ -22,10 +22,6 @@
 #ifndef ZSTD_FILEIO_ASYNCIO_H
 #define ZSTD_FILEIO_ASYNCIO_H
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
 #include "../lib/common/mem.h"     /* U32, U64 */
 #include "fileio_types.h"
 #include "platform.h"
@@ -35,6 +31,10 @@ extern "C" {
 
 #define MAX_IO_JOBS          (10)
 
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 typedef struct {
     /* These struct fields should be set only on creation and not changed afterwards */
     POOL_ctx* threadPool;