build: Add lib/dpkg/arch.c to POTFILES.in

Missed in commit 04c0362979a832bc0044d932cd1b705cef7b0d5b.

po/POTFILES.in

@@ -1,6 +1,7 @@
 # This is the list of all source files with translatable strings.
 
 lib/dpkg/ar.c
+lib/dpkg/arch.c
 lib/dpkg/buffer.c
 lib/dpkg/cleanup.c
 lib/dpkg/command.c