contrib: add SHA512SUM check test to restbed

Target .sum-restbed is missing from rules.mak and causes the execution error
of command "make fetch-all" in contrib.
Added the SHA512SUM file with sha512sum result on restbed archive to
call the default check SHA512 target.

Change-Id: Ic5b6b6b5bcf827cceb6999046cc9fa6917068c1e
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>

contrib/src/restbed/SHA512SUMS

+4262260c0b740e0be1c42e59e1680ab664b9e5b8f07e354b2bc66a7cdf29d738997011687689b3a25fdf365ddcb7861c8b9b0642053b7f5e956142973abfe6f5  restbed-4.0.tar.gz

contrib/src/restbed/rules.mak

@@ -47,3 +47,5 @@ restbed: restbed-$(RESTBED_VERSION).tar.gz
 	cd $< && cmake $(RESTBED_CONF) .
 	cd $< && $(MAKE) install
 	touch $@
+
+.sum-restbed: restbed-$(RESTBED_VERSION).tar.gz
\ No newline at end of file