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/zstdmt_compress.h

@@ -11,6 +11,11 @@
  #ifndef ZSTDMT_COMPRESS_H
  #define ZSTDMT_COMPRESS_H
 
+/* ===   Dependencies   === */
+#include "../common/zstd_deps.h"   /* size_t */
+#define ZSTD_STATIC_LINKING_ONLY   /* ZSTD_parameters */
+#include "../zstd.h"            /* ZSTD_inBuffer, ZSTD_outBuffer, ZSTDLIB_API */
+
  #if defined (__cplusplus)
  extern "C" {
  #endif
@@ -25,12 +30,6 @@
  *        otherwise ZSTDMT_createCCtx*() will fail.
  */
 
-/* ===   Dependencies   === */
-#include "../common/zstd_deps.h"   /* size_t */
-#define ZSTD_STATIC_LINKING_ONLY   /* ZSTD_parameters */
-#include "../zstd.h"            /* ZSTD_inBuffer, ZSTD_outBuffer, ZSTDLIB_API */
-
-
 /* ===   Constants   === */
 #ifndef ZSTDMT_NBWORKERS_MAX /* a different value can be selected at compile time */
 #  define ZSTDMT_NBWORKERS_MAX ((sizeof(void*)==4) /*32-bit*/ ? 64 : 256)