pkgsMusl.socat: fix build

Fix via Alpine.

pkgs/tools/networking/socat/default.nix

@@ -23,6 +23,11 @@ stdenv.mkDerivation rec {
       --replace /sbin/ifconfig ifconfig
   '';
 
+  configureFlags = lib.optionals stdenv.hostPlatform.isMusl [
+    # musl doesn't have getprotobynumber_r
+    "sc_cv_getprotobynumber_r=2"
+  ];
+
   buildInputs = [ openssl readline ];
 
   hardeningEnable = [ "pie" ];