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`

lib/common/zstd_trace.h

@@ -11,12 +11,12 @@
 #ifndef ZSTD_TRACE_H
 #define ZSTD_TRACE_H
 
+#include <stddef.h>
+
 #if defined (__cplusplus)
 extern "C" {
 #endif
 
-#include <stddef.h>
-
 /* weak symbol support
  * For now, enable conservatively:
  * - Only GNUC