GNU awk does not output escaped newlines in multi-line printc statements. This
leads to compile errors when trying to compile firmware(9) stubs created with
gawk, as multiple #include statements end up on the same line. Replace the
multi-line printc statement that outputs all of the #includes with one printc
per #include. This allows modules compatible with firmware(9) to be cross-built
from a Linux machine without requiring the one true awk to be installed.

I've intentionally done the minimal set of changes necessary to make gawk
produce valid (but not pretty) C code, to reduce the churn and keep fw_stubs.awk
as readable as possible.

Approved by: emaste (mentor)
MFC after: 2 weeks