ijar should use a hermetic zlib

I assume that the pre-existing ldflags setting is a simple translation
of linkopts = ["-lz"] from the source bazel BUILD file- which has a
comment saying it should be replaced because it's non-hermetic.  We have
a hermetic zlib in chromium, so let's use it.

Change-Id: I3b794b910be88ef34a8d5d13f4e06fb46f30e7c0
Reviewed-on: https://chromium-review.googlesource.com/c/1475188Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Mostyn Bramley-Moore <mostynb@vewd.com>
Cr-Commit-Position: refs/heads/master@{#632838}

third_party/ijar/BUILD.gn

@@ -20,11 +20,14 @@ if (current_toolchain == host_toolchain) {
       sources += [ "mapped_file_unix.cc" ]
     }
 
+    deps = [
+      "//third_party/zlib",
+    ]
+
     # Always build release since this is a build tool.
     if (is_debug) {
       configs -= [ "//build/config:debug" ]
       configs += [ "//build/config:release" ]
     }
-    ldflags = [ "-lz" ]
   }
 }