acpi/acpigen.c: Be explicit about char sign

The sign of 'char' is not standardized and with GCC is architecture
dependent.

This fixes warnings when compiling this file on arm64.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: I53b99835b2ffec5d752fc531fd59e4715f61aced
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76006
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/acpi/acpigen.c

@@ -187,7 +187,7 @@ void acpigen_write_name_unicode(const char *name, const char *string)
 	acpigen_write_len_f();
 	acpigen_write_integer(len);
 	for (size_t i = 0; i < len; i++) {
-		const char c = string[i];
+		const signed char c = string[i];
 		/* Simple ASCII to UTF-16 conversion, replace non ASCII characters */
 		acpigen_emit_word(c >= 0 ? c : '?');
 	}