More output is now produced on stdout by checkvintf.
Update logic to compare against the last line for incompatible.
Fixes the following tests:
test_check_target_files_vintf.CheckTargetFilesVintfTest#test_CheckVintf_kernel_incompat
test_check_target_files_vintf.CheckTargetFilesVintfTest#test_CheckVintf_matrix_incompat
test_check_target_files_vintf.CheckTargetFilesVintfTest#test_CheckVintf_sku_incompat
Bug: 251221029
Test: atest releasetools_test
Change-Id: I592acf9a3868e2a60e203b2201e7fadcbb0e7f04