remove unused header, which breaks the toolchain building

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

diff -Nur gcc-5.3.0.orig/libgcc/config/m68k/linux-atomic.c gcc-5.3.0/libgcc/config/m68k/linux-atomic.c
--- gcc-5.3.0.orig/libgcc/config/m68k/linux-atomic.c	2015-01-05 13:33:28.000000000 +0100
+++ gcc-5.3.0/libgcc/config/m68k/linux-atomic.c	2016-03-19 09:25:07.000000000 +0100
@@ -33,7 +33,6 @@
    using the kernel helper defined below.  There is no support for
    64-bit operations yet.  */
 
-#include <asm/unistd.h>
 #include <stdbool.h>
 
 #ifndef __NR_atomic_cmpxchg_32
