Fix T68911: "illegal recursive expansion of macro id"

I imagined that could be a problem for some drivers.
I should have avoided.

source/blender/draw/engines/select/shaders/selection_id_frag.glsl

@@ -1,14 +1,15 @@
 
 #ifdef UNIFORM_ID
 uniform int id;
-#  define id floatBitsToUint(intBitsToFloat(id))
+#  define _id floatBitsToUint(intBitsToFloat(id))
 #else
 flat in uint id;
+#  define _id id
 #endif
 
 out uint fragColor;
 
 void main()
 {
-  fragColor = id;
+  fragColor = _id;
 }