sds: use "pack" pragma instead of __attribute__(__packed__)
We need this to build flb_sds.c on MSVS without errors.
Note that GCC and clang also support the Microsoft `#pragma pack` feature,
so we can use Microsoft's syntax here without breaking portability.
typedef char *flb_sds_t;
+#pragma pack(push, 1)
struct flb_sds {
uint64_t len; /* used */
uint64_t alloc; /* excluding the header and null terminator */
char buf[];
-} __attribute__ ((__packed__));
+};
+#pragma pack(pop)
#define FLB_SDS_HEADER(s) ((struct flb_sds *) (s - FLB_SDS_HEADER_SIZE))
