Fix pr115388.c: plain char could be unsigned by default [PR115415]

Fix pr115388.c: plain char could be unsigned by default [PR115415]

This is a simple fix to the testcase as plain `char` could be
unsigned by default on some targets (e.g. aarch64 and powerpc).

Committed as obvious after quick test of the testcase on both aarch64 and x86_64.

gcc/testsuite/ChangeLog:

	PR testsuite/115415
	PR tree-optimization/115388
	* gcc.dg/torture/pr115388.c: Use `signed char` directly instead
	of plain `char`.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>