Makefile: Fix grep warning

grep (at least my version, grep-3.8-3.fc38.x86_64) warns with
"grep: warning: stray \ before -". Fix the warning by making
the command line input to grep less ambiguous.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>