strerror_override: add extern "C" and JSON_EXPORT specifiers for Visual C++ compilers

Fixes build on AppVeyor.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

strerror_override.h

@@ -4,10 +4,20 @@
 #include "config.h"
 #include <errno.h>
 
-char *_json_c_strerror(int errno_in);
+#include "json_object.h" /* for JSON_EXPORT */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+JSON_EXPORT char *_json_c_strerror(int errno_in);
 
 #ifndef STRERROR_OVERRIDE_IMPL
 #define strerror	_json_c_strerror
 #endif
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _json_strerror_override_h_ */