gard: fix installation rule

Use $(EXE) for the binary name instead of hard-coding. This completes
the "gard: Use consistent name" change from commit 41109e5 ("gard:
Use consistent name")

Fixes: 41109e5 ("gard: Use consistent name")
Signed-off-by: Dan Horák <dan@danny.cz>
[oliver: prefix in title, fixes tag]
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

external/gard/rules.mk

@@ -44,7 +44,7 @@ $(EXE): $(OBJS)
 	$(CC) $(LDFLAGS) $(CFLAGS) $^ -o $@
 
 install: all
-	install -D gard $(DESTDIR)$(sbindir)/opal-gard
-	install -D -m 0644 opal-gard.1 $(DESTDIR)$(mandir)/man1/opal-gard.1
+	install -D $(EXE) $(DESTDIR)$(sbindir)/$(EXE)
+	install -D -m 0644 $(EXE).1 $(DESTDIR)$(mandir)/man1/$(EXE).1