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

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>