package/ucl: fix build with host gcc 6.x

package/ucl: fix build with host gcc 6.x

ucl fails with the following error when using gcc 6.x on the host:

 checking whether your compiler passes the ACC conformance test... FAILED
 configure:
 configure: Your compiler failed the ACC conformance test - for details see
 configure: `config.log'. Please check that log file and consider sending
 configure: a patch or bug-report to <markus@oberhumer.com>.
 configure: Thanks for your support.
 configure:
 configure: error: ACC conformance test failed. Stop.

Fix the issue by using the ISO C90 standard.

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>