SPDY: use predefined constant for size of the Stream-ID field.

No functional changes.

src/http/ngx_http_spdy_filter_module.c

diff --git a/src/http/ngx_http_spdy_filter_module.c b/src/http/ngx_http_spdy_filter_module.c
index 8051776..4497e8b 100644
--- a/src/http/ngx_http_spdy_filter_module.c
+++ b/src/http/ngx_http_spdy_filter_module.c
@@ -751,7 +751,7 @@
             p = buf->start;
             buf->pos = p;
 
-            p += sizeof(uint32_t);
+            p += NGX_SPDY_SID_SIZE;
 
             (void) ngx_spdy_frame_write_flags_and_len(p, flags, len);