m68k: Switch to asm-generic/futex.h

As of commit 00f634bc522dedc8 ("asm-generic: add generic futex for
!CONFIG_SMP") asm-generic follows the m68k futex implementation.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
diff --git a/arch/m68k/include/asm/Kbuild b/arch/m68k/include/asm/Kbuild
index 9b6c691..1517ed1 100644
--- a/arch/m68k/include/asm/Kbuild
+++ b/arch/m68k/include/asm/Kbuild
@@ -6,6 +6,7 @@
 generic-y += emergency-restart.h
 generic-y += errno.h
 generic-y += exec.h
+generic-y += futex.h
 generic-y += hw_irq.h
 generic-y += ioctl.h
 generic-y += ipcbuf.h