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/compress/zstd_double_fast.h

@@ -11,13 +11,13 @@
 #ifndef ZSTD_DOUBLE_FAST_H
 #define ZSTD_DOUBLE_FAST_H
 
+#include "../common/mem.h"      /* U32 */
+#include "zstd_compress_internal.h"     /* ZSTD_CCtx, size_t */
+
 #if defined (__cplusplus)
 extern "C" {
 #endif
 
-#include "../common/mem.h"      /* U32 */
-#include "zstd_compress_internal.h"     /* ZSTD_CCtx, size_t */
-
 #ifndef ZSTD_EXCLUDE_DFAST_BLOCK_COMPRESSOR
 
 void ZSTD_fillDoubleHashTable(ZSTD_matchState_t* ms,