Fixed warning during nginx.xs compilation.

src/http/modules/perl/nginx.xs

diff --git a/src/http/modules/perl/nginx.xs b/src/http/modules/perl/nginx.xs
index ecd11ff..ed97439 100644
--- a/src/http/modules/perl/nginx.xs
+++ b/src/http/modules/perl/nginx.xs
@@ -476,7 +476,7 @@
     }
 
     if (header->key.len == sizeof("Content-Encoding") - 1
-        && ngx_strncasecmp(header->key.data, "Content-Encoding",
+        && ngx_strncasecmp(header->key.data, (u_char *) "Content-Encoding",
                            sizeof("Content-Encoding") - 1) == 0)
     {
         r->headers_out.content_encoding = header;