header-abi-diff: Fix return code for extension in presence of elf only changes.

Before this change, an elf symbol change not exported through public
headers would short circuit the check for api extension. As a result,
header-abi-diff would return a compatible return status (0). This isn't
correct, extensions should take precedence over elf only changes and an
extension status should be returned (non zero).

Test: tests/test.py

Change-Id: I6907d0ebaf332d019cdf2e264316f7d521e00d82