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>

gcc/testsuite/gcc.dg/torture/pr115388.c

@@ -2,7 +2,7 @@
 
 int printf(const char *, ...);
 int a[10], b, c, d[0], h, i, j, k, l;
-char e = -1, g;
+signed char e = -1, g;
 volatile int f;
 static void n() {
   while (e >= 0)