Makefile: Fix `verilator` variable (#427)

Signed-off-by: Wilson Snyder <wsnyder@wsnyder.org>

Makefile

@@ -34,7 +34,7 @@ BOARD          ?= genesys2
 mkfile_path := $(abspath $(lastword $(MAKEFILE_LIST)))
 root-dir := $(dir $(mkfile_path))
 
-support_verilator_4 := $(shell (verilator --version | grep '4\.') &> /dev/null; echo $$?)
+support_verilator_4 := $(shell ($(verilator) --version | grep '4\.') &> /dev/null; echo $$?)
 ifeq ($(support_verilator_4), 0)
 	verilator_threads := 2
 endif