osdep: Introduce include/grub/osdep/major.h and use it

... to factor out fix for glibc 2.25 introduced in 7a5b301e3 (build: Use
AC_HEADER_MAJOR to find device macros).

Note: Once glibc 2.25 is old enough and this fix is not needed also
AC_HEADER_MAJOR in configure.ac should be removed.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/osdep/devmapper/getroot.c

@@ -40,12 +40,7 @@
 #include <limits.h>
 #endif
 
-#if defined(MAJOR_IN_MKDEV)
-#include <sys/mkdev.h>
-#elif defined(MAJOR_IN_SYSMACROS)
-#include <sys/sysmacros.h>
-#endif
-
+#include <grub/osdep/major.h>
 #include <libdevmapper.h>
 
 #include <grub/types.h>