x86/build: Permit building with old make versions

At least Make 3.82 dislikes the tab in front of the $(warning) function:

  arch/x86/Makefile:162: *** recipe commences before first target.  Stop.

Let's be gentle.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1944fcd8-e3df-d1f7-c0e4-60aeb1917a24@siemens.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Makefile

@@ -159,7 +159,7 @@ ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	# If '-Os' is enabled, disable it and print a warning.
         ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
           undefine CONFIG_CC_OPTIMIZE_FOR_SIZE
-	  $(warning Disabling CONFIG_CC_OPTIMIZE_FOR_SIZE.  Your compiler does not have -mfentry so you cannot optimize for size with CONFIG_FUNCTION_GRAPH_TRACER.)
+          $(warning Disabling CONFIG_CC_OPTIMIZE_FOR_SIZE.  Your compiler does not have -mfentry so you cannot optimize for size with CONFIG_FUNCTION_GRAPH_TRACER.)
         endif
 
     endif