Remove extern C blocks from lib/* internal APIs (except xxhash.h)

lib/common/error_private.h

@@ -21,10 +21,6 @@
 #include "debug.h"
 #include "zstd_deps.h"       /* size_t */
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
 /* ****************************************
 *  Compiler-specific
 ******************************************/
@@ -159,8 +155,4 @@ void _force_has_format_string(const char *format, ...) {
         }                                                                          \
     } while(0)
 
-#if defined (__cplusplus)
-}
-#endif
-
 #endif /* ERROR_H_MODULE */