[testsuite, Android] Add <stdlib.h> to pr56407.c

[Andreas Schwab]

Alexander Ivchenko <aivchenk@gmail.com> writes:

> Still, in C standart it is said that rand is defined in stdlib.h, and
> we don't include it in that testcase.

It also says: "Provided that a library function can be declared without
reference to any type defined in a header, it is also permissible to
declare the function and use it without including its associated
header."

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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