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`

zlibWrapper/zstd_zlibwrapper.h

@@ -11,11 +11,6 @@
 #ifndef ZSTD_ZLIBWRAPPER_H
 #define ZSTD_ZLIBWRAPPER_H
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
-
 #define ZLIB_CONST
 #define Z_PREFIX
 #define ZLIB_INTERNAL   /* disables gz*64 functions but fixes zlib 1.2.4 with Z_PREFIX */
@@ -29,6 +24,11 @@ extern "C" {
     #define _Z_OF OF
 #endif
 
+
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 /* returns a string with version of zstd library */
 const char * zstdVersion(void);