lib: Improve the is_signed_type() kunit test

Since the definition of is_signed_type() has been moved from
<linux/overflow.h> to <linux/compiler.h>, include the latter header file
instead of the former. Additionally, add a test for the type 'char'.

Cc: Isabella Basso <[e-mail]>
Cc: Rasmus Villemoes <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

lib/is_signed_type_kunit.c

···
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <kunit/test.h>
-#include <linux/overflow.h>
+#include <linux/compiler.h>
 
 enum unsigned_enum {
 	constant_a = 3,
···
 	KUNIT_EXPECT_EQ(test, is_signed_type(bool), false);
 	KUNIT_EXPECT_EQ(test, is_signed_type(signed char), true);
 	KUNIT_EXPECT_EQ(test, is_signed_type(unsigned char), false);
+#ifdef __CHAR_UNSIGNED__
+	KUNIT_EXPECT_EQ(test, is_signed_type(char), false);
+#else
+	KUNIT_EXPECT_EQ(test, is_signed_type(char), true);
+#endif
 	KUNIT_EXPECT_EQ(test, is_signed_type(int), true);
 	KUNIT_EXPECT_EQ(test, is_signed_type(unsigned int), false);
 	KUNIT_EXPECT_EQ(test, is_signed_type(long), true);