usb_modeswitch: fix static build failure

abort() is used in two different places, jim (proper C library abort)
and usb_modeswitch.c (custom abort function).
Status: emailed the patch to author.

Fixes:
http://autobuild.buildroot.net/results/789/7899a37d2c766716a1deff58e8be94c5c4962240/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>