ubihealthd: Build only if sys/random.h is present

The ubihealthd depends on sys/random.h , which is not present on some
older systems. Build ubihealthd only if sys/random.h is present.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>