generate_packets.py: don't indent preprocessor directives
Part of #43927. For better readability (and thus debuggability) of the generated code, preprocessor directives should not be indented (to keep in style with the non-generated codebase). Make the prefix() function not indent lines starting with a #.
This patch also makes it so blank lines are no longer indented, something that I (mistakenly) thought had already been the case.
Part of #43927. For better readability (and thus debuggability) of the generated code, preprocessor directives should not be indented (to keep in style with the non-generated codebase). Make the prefix() function not indent lines starting with a #.