linuxptp: Fix cross-compilation

Linux-PTP uses a kernel-like build system which requires that the
`CROSS_COMPILE` variable be set to the target prefix when
cross-compiling.

pkgs/os-specific/linux/linuxptp/default.nix

@@ -15,7 +15,10 @@ stdenv.mkDerivation rec {
        '/usr/include/linux/' "${linuxHeaders}/include/linux/"
   '';
 
-  makeFlags = [ "prefix=" ];
+  makeFlags = [
+    "prefix="
+    "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
+  ];
 
   preInstall = ''
     export DESTDIR=$out