Mp4: fixed build on win32 after r4689.

src/http/modules/ngx_http_mp4_module.c

diff --git a/src/http/modules/ngx_http_mp4_module.c b/src/http/modules/ngx_http_mp4_module.c
index eb51ab2..d3be234 100644
--- a/src/http/modules/ngx_http_mp4_module.c
+++ b/src/http/modules/ngx_http_mp4_module.c
@@ -1024,7 +1024,7 @@
                          + NGX_HTTP_MP4_MOOV_BUFFER_EXCESS * no_mdat;
     }
 
-    if (ngx_http_mp4_read(mp4, atom_data_size) != NGX_OK) {
+    if (ngx_http_mp4_read(mp4, (size_t) atom_data_size) != NGX_OK) {
         return NGX_ERROR;
     }