struct_test: use explicit signed char for Array4Chars field

Changes the Array4Chars struct field from char to signed char to ensure
consistent signedness across platforms. The signedness of plain char is
implementation-defined and can vary between compilers and platforms,
potentially causing test inconsistencies.

Using signed char makes the signedness explicit and ensures consistent
behavior in struct marshalling tests across Darwin and Linux platforms.

Author: tmc

testdata/structtest/struct_test.c

@@ -280,7 +280,7 @@ unsigned int Array2UnsignedShorts(struct Array2UnsignedShort a) {
 }
 
 struct Array4Chars {
-    char a[4];
+    signed char a[4];
 };
 
 int Array4Chars(struct Array4Chars a) {