Allow building CHIP without lwip checked out

This used to work prior to project-chip#2325, which added an unconditional
dependency on lwip even if we're using sockets. Make it conditional.

src/inet/BUILD.gn

@@ -20,6 +20,7 @@ import("//build_overrides/nlio.gni")
 import("${chip_root}/gn/chip/buildconfig_header.gni")
 import("${chip_root}/gn/chip/tests.gni")
 import("${chip_root}/src/platform/device.gni")
+import("${chip_root}/src/system/system.gni")
 import("inet.gni")
 
 declare_args() {
@@ -97,11 +98,14 @@ static_library("inet") {
   public_deps = [
     ":inet_config_header",
     "${chip_root}/src/lib/support",
-    "${chip_root}/src/lwip",
     "${chip_root}/src/system",
     "${nlio_root}:nlio",
   ]
 
+  if (chip_system_config_use_lwip) {
+    public_deps += [ "${chip_root}/src/lwip" ]
+  }
+
   if (chip_inet_config_enable_raw_endpoint) {
     sources += [
       "RawEndPoint.cpp",