[Hdx] fixed GLSL const qualifier errors

A const qualifier is not allowed on function return values
in GLSL. The NVIDIA shader compiler apparently allows this
but it is a compile time error on AMD and other compilers.

(Internal change: 2116327)

Author: davidgyu

pxr/imaging/hdx/shaders/selection.glslfx

@@ -35,7 +35,7 @@
 /// selection state.
 
 // This should match the HdSelection enum.
-const int GetMaxNumSelectionHighlightModes()
+int GetMaxNumSelectionHighlightModes()
 {
     return 2;
 }
@@ -365,7 +365,7 @@ bool IsSelected()
 --- --------------------------------------------------------------------------
 -- glsl Selection.Geometry.WireSelOffset
 
-const int GetMaxNumSelectionHighlightModes();
+int GetMaxNumSelectionHighlightModes();
 bool IsElementSelected(int mode);
 int GetElementID(); // code gen
 
@@ -506,4 +506,4 @@ vec4 ApplySelectionColor(vec4 color)
 
 #endif
     return color;
-}
+}