autoconf: Fix strict prototype errors in generated tests

This will fix issues with autoconf tests which fail due to lacking
prototypes

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>