debian: Add gbp.conf

This makes, tagging, importing, etc easier
4 jobs for gbp.conf in 6 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Package
passed #358106
librem5
package-pureos-byzantium

00:01:27

passed #358107
librem5:arm64
package-pureos-byzantium:arm64

00:02:39

 
  Test Package
passed #358108
librem5
autopkgtest-pureos-byzantium-package

00:01:11

passed #358109
librem5
lintian-pureos-byzantium-package

00:01:01