abuild: add ignore-missing-so-files option

After commit 3fa9fda0, trace-apk-deps would return non-zero if any of the auto-detected .so files were not found. For the use case where the author of the APKBUILD file specifies dependencies for external .so files explicitly, but still wants trace-apk-deps to auto-detect system libraries, add a ignore-missing-so-files option.