[gdb/build] Add GPL header comment to gdb/features/feature_to_c.awk

Commit 97033da5 ("[gdb/build] Cleanup gdb/features/feature_to_c.sh")
factored out new file gdb/features/feature_to_c.awk out of
gdb/features/feature_to_c.sh, but failed to add the GPL header comment, so add
this now.

Tested on x86_64-linux.