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/benchzstd.h

@@ -14,10 +14,6 @@
   * and display progress result and final summary
   */
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
 #ifndef BENCH_ZSTD_H_3242387
 #define BENCH_ZSTD_H_3242387
 
@@ -27,6 +23,10 @@ extern "C" {
 #include "../lib/zstd.h"     /* ZSTD_compressionParameters */
 
 
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 /* ===  Constants  === */
 
 #define MB_UNIT 1000000