linuxPackages*.perf: fix build with default gcc

Broken since 9842a107.

pkgs/os-specific/linux/kernel/perf.nix

@@ -32,6 +32,9 @@ stdenv.mkDerivation {
   # bad `ld' and other stuff.
   NIX_CFLAGS_COMPILE = [
     "-Wno-error=cpp" "-Wno-error=bool-compare" "-Wno-error=deprecated-declarations"
+  ]
+    # gcc before 6 doesn't know these options
+    ++ stdenv.lib.optionals (hasPrefix "gcc-6" stdenv.cc.cc) [
       "-Wno-error=unused-const-variable" "-Wno-error=misleading-indentation"
     ];