resolve c++ compilation issue

Synchronize with obieq/sds#1
Pending in antirez/sds#125

Author: bitbashing

sds.h

@@ -80,7 +80,11 @@ struct __attribute__ ((__packed__)) sdshdr64 {
 #define SDS_TYPE_64 4
 #define SDS_TYPE_MASK 7
 #define SDS_TYPE_BITS 3
+#ifdef __cplusplus
+#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (struct sdshdr##T*)((s)-(sizeof(struct sdshdr##T)));
+#else
 #define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
+#endif
 #define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
 #define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)