Fix builds with C23 compilers
Relies on C99 & later stdbool.h header to avoid making our own definitions of bool, true, & false that C23 already defines for us.
./VBoxVideoIPRT.h:35:5: error: expected identifier before ‘false’
35 | false = 0,
| ^~~~~
./VBoxVideoIPRT.h:37:3: error: expected ‘;’, identifier or ‘(’ before ‘bool’
37 | } bool;
| ^~~~
./VBoxVideoIPRT.h:37:3: warning: useless type name in empty declaration