libio.h is not provided by uClibc, in order to be able to test the
definition of __UCLIBC__ we need to include stdlib.h, which also
includes stddef.h, providing the definition of 'NULL'.
Signed-off-by: Florian Fainelli <florian@openwrt.org> Signed-off-by: Will Deacon <will.deacon@arm.com>