bsd.compat.mk: A few cosmetic fixes.

• Add a missing ')' to a warning.

• Consistently use {} when expanding variables.

• Remove a spurious blank line.

Reviewed by: imp, emaste
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D34172

(cherry picked from commit ffe74ab77f234fabe691bd79e254cd000e07dfe0)