main/reboot-mode: Fix printing of error (!551)

Previously, running reboot-mode as an unprivileged user resulted in

    Error: No error information

without a newline at the end. According to SYSCALL(2), the return value
of -1 indicated an error, but the actual error code is stored in errno.

main/reboot-mode/APKBUILD

 # Contributor: Daniele Debernardi <drebrez@gmail.com>
 # Maintainer: Daniele Debernardi <drebrez@gmail.com>
 pkgname=reboot-mode
-pkgver=0.1
+pkgver=0.2
 pkgrel=0
 pkgdesc="Reboot the device to a specific mode"
 url="https://postmarketos.org"
@@ -22,4 +22,4 @@ package() {
 		"${pkgdir}/usr/sbin/reboot-mode"
 }
 
-sha512sums="37d282608f30b1b8fa4f7501a8afcaa81902c9dfaee0c4c92ab05028e3a99a69c601dc7fce6ab06582db044b6fd0f5bffabe9d6064af9e670266978d66fe2515  reboot-mode.c"
+sha512sums="a0cd4097d24fb6c66edce60558942d18ccf45925013013e5af5437471b1a855abe12857deb45b5f970b04e8b9dba99177b9080fff584fbd16262f0a7aabbf4b1  reboot-mode.c"

main/reboot-mode/reboot-mode.c

@@ -4,6 +4,7 @@
  *
  * Copyright (C) 2019 Daniele Debernardi <drebrez@gmail.com>
  */
+#include <errno.h>
 #include <linux/reboot.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -42,8 +43,8 @@ int main(int argc, char **argv)
 	int ret;
 	ret = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_RESTART2, argv[1]);
 
-	if (ret) {
-		printf("Error: %s", strerror(ret));
+	if (ret < 0) {
+		printf("Error: %s\n", strerror(errno));
 	}
 
 	return ret;