Makefile: Make make get-git work under OpenBSD.

OpenBSD tar(1) defaults to read from "/dev/rst0" when not specifying an
filename and thus fails to extract the Git sourcecode when not passing
stdin as input file descriptor explicitly.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -168,4 +168,4 @@ clean-doc:
 	rm -f cgitrc.5 cgitrc.5.html cgitrc.5.pdf cgitrc.5.xml cgitrc.5.fo
 
 get-git:
-	curl $(GIT_URL) | tar -xj && rm -rf git && mv git-$(GIT_VER) git
+	curl $(GIT_URL) | tar -xjf - && rm -rf git && mv git-$(GIT_VER) git