gstreamer: fix xtensa configure

Disable unaligned access to avoid configure run test.

Fixes
http://autobuild.buildroot.net/results/a0c/a0cca019b0b2f33b6c2be17e5a44a807bbb4dac1/

.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>