x86/build: Annotate die() with noreturn to fix build warning on clang

Fixes below warning with clang:

  In file included from ../arch/x86/tools/relocs_64.c:17:
  ../arch/x86/tools/relocs.c:977:6: warning: variable 'do_reloc' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20161126222229.673-1-pefoley2@pefoley.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/tools/relocs.h

@@ -16,7 +16,7 @@
 #include <regex.h>
 #include <tools/le_byteshift.h>
 
-void die(char *fmt, ...);
+void die(char *fmt, ...) __attribute__((noreturn));
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))