Strict C99: make sure that fileno() is declared
Strict C99: make sure that fileno() is declared

<stdio.h> only declares the non-ISO-C function fileno() if an
appropriate POSIX symbol is defined or if using a compiler such as GCC
in non-pedantic mode. Define the appropriate POSIX symbol.

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

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