glsl: Silence may unused parameter warnings in glsl/ir.h

Every file that included glsl/ir.h had a warning like:

src/compiler/glsl/ir.h: In member function ‘virtual bool ir_rvalue::is_lvalue(const _mesa_glsl_parse_state*) const’:
src/compiler/glsl/ir.h:236:64: warning: unused parameter ‘state’ [-Wunused-parameter]
    virtual bool is_lvalue(const struct _mesa_glsl_parse_state *state = NULL) const
                                                                ^
Cc: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Fixes: fa4ebf6b8d ("glsl: add _mesa_glsl_parse_state object to is_lvalue()")
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/glsl/ir.h

@@ -233,7 +233,7 @@ public:
 
    ir_rvalue *as_rvalue_to_saturate();
 
-   virtual bool is_lvalue(const struct _mesa_glsl_parse_state *state = NULL) const
+   virtual bool is_lvalue(const struct _mesa_glsl_parse_state * = NULL) const
    {
       return false;
    }