Include cstddef in the header for C++

So C++ compiler always has access to the definition of size_t.

Signed-off-by: Oleksandr Kravchuk <open.source@oleksandr-kravchuk.com>

src/libuboot.h

@@ -6,6 +6,8 @@
  */
 
 #ifdef __cplusplus
+#include <cstddef>
+
 extern "C" {
 #endif