Blackfin: merge asm/mutex.h into kbuild too

This header was being rewritten while the asm-generic kbuild support
was in flight, so it missed out on the update. Punt the stub and use
the kbuild now that everything has settled.

Signed-off-by: Mike Frysinger <[e-mail]>

arch/blackfin/include/asm/Kbuild

···
 generic-y += local.h
 generic-y += mman.h
 generic-y += msgbuf.h
+generic-y += mutex.h
 generic-y += param.h
 generic-y += percpu.h
 generic-y += pgalloc.h

arch/blackfin/include/asm/mutex.h

···
-#include <asm-generic/mutex-dec.h>