efi/libstub: Fix 4.9.139 merge resolution

Commit 6b66b2d8 ("efi/libstub: Preserve .debug sections after
absolute relocation check") removed the '-g0' from cflags.

Change-Id: I8f5cfbc215d3697e3c12a4bc4d3b5f96f33fd586
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>