Fix contracts-tmpl-spec2.C on targets where plain char is unsigned by default

Fix contracts-tmpl-spec2.C on targets where plain char is unsigned by default

Since contracts-tmpl-spec2.C is just testing contracts, I thought it would be better
to just add `-fsigned-char` to the options rather than change the testcase to support
both cases.

Committed after testing on aarch64-linux-gnu.

gcc/testsuite/ChangeLog:

	PR testsuite/108321
	* g++.dg/contracts/contracts-tmpl-spec2.C: Add -fsigned-char
	to options.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>
(cherry picked from commit 6e15e4e1abed02443a27a69455f4bfa49457c99e)