[modula2] Add missing stdbool to testcode

The type bool is now used to represent BOOLEAN.  The C source
must now include <stdbool.h>.

gcc/testsuite/ChangeLog:

	* gm2/types/run/pass/d.c: Add missing include <stdbool.h>.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>