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

gtests/Spv.FromFile.cpp

@@ -450,6 +450,7 @@ INSTANTIATE_TEST_CASE_P(
         "spv.xfbOffsetOnBlockMembersAssignment.vert",
         "spv.xfbOffsetOnStructMembersAssignment.vert",
         "spv.xfbOverlapOffsetCheckWithBlockAndMember.vert",
+        "spv.xfbStrideJustOnce.vert",
     })),
     FileNameAsCustomTestSuffix
 );