cross-build: progname.c is used only on linux, so no ifdefs needed

The logic in the makefile ensures this is only present on Linux, so no
ifdef is needed. I overlooked comments to the effect in the original
pull request somehow.

Fixes: 0e03402139da
Suggested by: arichards, jrtc27
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D45374