ada: Add an assert in Posix Interrupt_Wait

ada: Add an assert in Posix Interrupt_Wait

All functions but Interrupt_Wait in s-inmaop__posix are checking the
result of their syscalls with an assert. However, any return code of
sigwait different than 0 means that something went wrong for it.

From sigwait man:
> RETURN VALUE
>      On success, sigwait() returns 0.  On  error,  it  returns  a
>      positive error number (listed in ERRORS).

gcc/ada/

	* libgnarl/s-inmaop__posix.adb: Add assert after sigwait in
	Interrupt_Wait