Add ICU to static (input-output-hk#1945)

* Add ICU to static

This adds the icu package to the musl static set. Needed for example for cardano-db-sync.

Author: angerman

overlays/musl.nix

@@ -25,8 +25,11 @@ final: prev: prev.lib.optionalAttrs prev.stdenv.hostPlatform.isMusl ({
   # See https://github.com/input-output-hk/haskell.nix/issues/948
   postgresql = (prev.postgresql.overrideAttrs (old: { dontDisableStatic = true; }))
     .override { enableSystemd = false; gssSupport = false; };
+  
   openssl = prev.openssl.override { static = true; };
 
+  icu = (prev.icu.overrideAttrs (old: { configureFlags = old.configureFlags ++ [ "--enable-static" "--disable-shared" ]; }));
+
   # Fails on cross compile
   nix = prev.nix.overrideAttrs (_: { doInstallCheck = false; });
 } // prev.lib.optionalAttrs (prev.lib.versionAtLeast prev.lib.trivial.release "20.03") {