misc/cgo/test: use char, not int, so test works on big-endian systems

Updates #32579
Fixes #32770

Change-Id: I32d1dea7505e8ad22e11a9806e10d096924b729b
Reviewed-on: https://go-review.googlesource.com/c/go/+/183778
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

misc/cgo/test/test.go

@@ -853,7 +853,7 @@ static void issue29781F(char **p, int n) {}
 static uint16_t issue31093F(uint16_t v) { return v; }
 
 // issue 32579
-typedef struct S32579 { int data[1]; } S32579;
+typedef struct S32579 { unsigned char data[1]; } S32579;
 */
 import "C"