header-abi-diff: Fix omission of warning message for extensions.

Bug: 79928919

Test: Without change : extend libjpeg; build breaks without warning message
      specifying extension

Test: With change : extend libjpeg; build breaks with warning message specifying
      extension

Test: tests/test.py

Merged-In: I1653a977d0945810b4b9dc4ec9923b12c4f5dc36
Change-Id: I1653a977d0945810b4b9dc4ec9923b12c4f5dc36
(cherry picked from commit b31dbb37)