meson: require `nm` again on Unix systems

This was made optional in ff9bf223c24143260a97 ("meson: make nm binary optional")
for Windows, but proper windows has been added and `nm` is now only used
on Unix systems.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviwed-by: Dylan Baker <dylan@pnwbakers>
1 file changed