Fixed build without atomic operations.

src/core/ngx_shmtx.c

diff --git a/src/core/ngx_shmtx.c b/src/core/ngx_shmtx.c
index 5a9242d..3414868 100644
--- a/src/core/ngx_shmtx.c
+++ b/src/core/ngx_shmtx.c
@@ -299,10 +299,10 @@
 }
 
 
-void
+ngx_uint_t
 ngx_shmtx_force_unlock(ngx_shmtx_t *mtx, ngx_pid_t pid)
 {
-    /* void */
+    return 0;
 }
 
 #endif