kbuild: explicit enable framepointer

Newer gcc versions automatically turns on -fomit-frame-pointer when
-O2 is specified thus breaking CONFIG_FRAME_POINTER option.
Explicitly specifying -fno-omit-frame-pointer fixes it.

From: Tejun Heo <tj@home-tj.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -498,7 +498,9 @@ else
 CFLAGS		+= -O2
 endif
 
-ifndef CONFIG_FRAME_POINTER
+ifdef CONFIG_FRAME_POINTER
+CFLAGS		+= -fno-omit-frame-pointer
+else
 CFLAGS		+= -fomit-frame-pointer
 endif