make: Fixed prettyasserts prefix -Plfs_ -> -Plfs3_

This was missing from the big lfs -> lfs3 rename, probably because it
didn't actually break testing. It just prevents prettyasserts from
making LFS3_ASSERT pretty.

There's already been a bunch of benchmarking targeting the current hash,
and we're probably going to find other missed prefixes in corners of the
codebase anyways, so I'm not going to bother rebasing.

Makefile

@@ -646,10 +646,10 @@ $(BUILDDIR)/%.s: $(BUILDDIR)/%.c
 	$(CC) -S $(CFLAGS) $< -o$@
 
 $(BUILDDIR)/%.a.c: %.c
-	$(PRETTYASSERTS) -Plfs_ $< -o$@
+	$(PRETTYASSERTS) -Plfs3_ $< -o$@
 
 $(BUILDDIR)/%.a.c: $(BUILDDIR)/%.c
-	$(PRETTYASSERTS) -Plfs_ $< -o$@
+	$(PRETTYASSERTS) -Plfs3_ $< -o$@
 
 $(BUILDDIR)/%.t.c: %.toml
 	./scripts/test.py -c $< $(TESTCFLAGS) -o$@