utils: make sure the common utility library gets compiled c99
utils: make sure the common utility library gets compiled c99

This will reduce some warnings on older versions of gcc

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio