Add a missing include needed for glibc 2.28 to avoid:

| ../../git/tools/backlight_helper.c: In function 'main':
| ../../git/tools/backlight_helper.c:54:34: error: implicit declaration of function 'major' [-Werror=implicit-function-declaration]
|   if (fd < 0 || fstat(fd, &st) || major(st.st_dev))
|                                   ^~~~~
| ../../git/tools/backlight_helper.c:54:34: warning: nested extern declaration of 'major' [-Wnested-externs]
| cc1: some warnings being treated as errors
| Makefile:666: recipe for target 'backlight_helper.o' failed

Upstream-Status: Pending
RP 2018/8/12

Index: git/tools/backlight_helper.c
===================================================================
--- git.orig/tools/backlight_helper.c
+++ git/tools/backlight_helper.c
@@ -8,6 +8,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 
 #if MAJOR_IN_MKDEV
 #include <sys/mkdev.h>