Fix missing compile error when using a function as a variable

oslc compiles this without error:
```
float some_function(float f)
{
    return f;
}

shader test()
{
    float f = some_function;
}
```

but when run with testshade it would give this:
```
ERROR: Parsing shader test: unknown arg some_function
src/liboslexec/loadshader.cpp:389: Failed assertion 'm_nargs == i'
```