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`

contrib/seekable_format/zstd_seekable.h

@@ -1,13 +1,13 @@
 #ifndef SEEKABLE_H
 #define SEEKABLE_H
 
+#include <stdio.h>
+#include "zstd.h"   /* ZSTDLIB_API */
+
 #if defined (__cplusplus)
 extern "C" {
 #endif
 
-#include <stdio.h>
-#include "zstd.h"   /* ZSTDLIB_API */
-
 
 #define ZSTD_seekTableFooterSize 9