We're making an NDK API Council +1 a requirment for changes to these
files, so we need to know that all symbol files will match a given
file pattern.
Test: Changed a symbol file to foo.txt, `make ndk` failed
Test: make ndk
Bug: http://b/35870541
Change-Id: I29690fb97746ad682e57845fea3f1114cfd77598