GlslangToSpv: prevent add XfbStride twice

Without this commit, if the XfbStride was explicitly set, the
decoration was added twice on the shader.

v2 (changes after Jonh Kessenich first review)
   * Simplified by just removing the firs assignment
   * Removed assert

Test/spv.xfbStrideJustOnce.vert

@@ -0,0 +1,14 @@
+#version 450
+
+layout(xfb_buffer=2) out;
+
+layout(location=5, xfb_stride=20) out block {
+   float y1_out;
+   vec4 y2_out;
+};
+
+void main() {
+   y1_out = 7.0;
+   y2_out = vec4(1.0, 0.0, 0.0, 1.0);
+   gl_Position = vec4(0.0);
+}