glsl2: Initialize ir_function_signature::is_built_in.

Fixes a valgrind error.

src/glsl/ir.cpp

@@ -837,6 +837,7 @@ ir_function_signature::ir_function_signature(const glsl_type *return_type)
    : return_type(return_type), is_defined(false), _function(NULL)
 {
    this->ir_type = ir_type_function_signature;
+   this->is_built_in = false;
 }