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/bitstream.h

@@ -14,9 +14,6 @@
 #ifndef BITSTREAM_H_MODULE
 #define BITSTREAM_H_MODULE
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
 /*
 *  This API consists of small unitary functions, which must be inlined for best performance.
 *  Since link-time-optimization is not available for all compilers,
@@ -32,7 +29,6 @@ extern "C" {
 #include "error_private.h"  /* error codes and messages */
 #include "bits.h"           /* ZSTD_highbit32 */
 
-
 /*=========================================
 *  Target specific
 =========================================*/
@@ -44,6 +40,10 @@ extern "C" {
 #  endif
 #endif
 
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 #define STREAM_ACCUMULATOR_MIN_32  25
 #define STREAM_ACCUMULATOR_MIN_64  57
 #define STREAM_ACCUMULATOR_MIN    ((U32)(MEM_32bits() ? STREAM_ACCUMULATOR_MIN_32 : STREAM_ACCUMULATOR_MIN_64))