fix OpenBSD building, broken in r2616

src/os/unix/ngx_posix_config.h

diff --git a/src/os/unix/ngx_posix_config.h b/src/os/unix/ngx_posix_config.h
index 152e232..3644151 100644
--- a/src/os/unix/ngx_posix_config.h
+++ b/src/os/unix/ngx_posix_config.h
@@ -44,6 +44,9 @@
 #include <grp.h>
 #include <dirent.h>
 #include <glob.h>
+#if (NGX_HAVE_SYS_PARAM_H)
+#include <sys/param.h>          /* statfs() */
+#endif
 #if (NGX_HAVE_SYS_MOUNT_H)
 #include <sys/mount.h>          /* statfs() */
 #endif