Update Makefrag-verilator
Changed verilator version from 3.904 to 3.920, which fixes a bug that prevented the default example to compile correctly
# Build and install our own Verilator, to work around versionining issues.
-VERILATOR_VERSION=3.904
+VERILATOR_VERSION=3.920
VERILATOR_SRCDIR=verilator/src/verilator-$(VERILATOR_VERSION)
INSTALLED_VERILATOR=$(abspath verilator/install/bin/verilator)
$(INSTALLED_VERILATOR): $(VERILATOR_SRCDIR)/bin/verilator
