Fix depfile in otatoolsPackageCertRule

The depfile dependencies must all be on the same line as the output
file. (or escape the newlines with backslashes)

Bug: 395988167
Test: m otatools_package_cert_files(-soong if on soong+make builds), observe it builds. Rerun command, observe it doesn't build. touch packages/modules/adb/apex/com.android.adbd.avbpubkey and rerun command, observe it builds again.
Change-Id: I2b2ee397b406488797e16b04880d0396784029fe