darwin.top: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
clang-11. Otherwise build fails as:

    duplicate symbol '_tsamp' in: main.o top.o

pkgs/os-specific/darwin/apple-source-releases/top/default.nix

@@ -3,6 +3,9 @@
 appleDerivation {
   nativeBuildInputs = [ xcbuildHook ];
   buildInputs = [ apple_sdk.frameworks.IOKit ncurses libutil ];
+  # Workaround build failure on -fno-common toolchains:
+  #   duplicate symbol '_tsamp' in: main.o top.o
+  NIX_CFLAGS_COMPILE = "-fcommon";
   NIX_LDFLAGS = "-lutil";
   installPhase = ''
     install -D Products/Release/libtop.a $out/lib/libtop.a