git-credential-netrc: make "all" default target of Makefile

Running "make" in contrib/credential/netrc should run the "all" target
rather than the "test" target.  Add an empty "all::" target like most of
our other Makefiles.
Signed-off-by: NTodd Zullinger <tmz@pobox.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/credential/netrc/Makefile

+# The default target of this Makefile is...
+all::
+
 test:
 	./t-git-credential-netrc.sh