Meta: Add LibJS and its dependencies to gn build

Meta/gn/secondary/Userland/Libraries/LibRegex/BUILD.gn

@@ -0,0 +1,19 @@
+shared_library("LibRegex") {
+  output_name = "regex"
+  include_dirs = [ "//Userland/Libraries" ]
+  sources = [
+    "RegexByteCode.cpp",
+    "RegexLexer.cpp",
+    "RegexMatcher.cpp",
+    "RegexOptimizer.cpp",
+    "RegexParser.cpp",
+  ]
+  if (current_os == "serenity") {
+    sources += [ "C/Regex.cpp" ]
+  }
+  deps = [
+    "//AK",
+    "//Userland/Libraries/LibCore",
+    "//Userland/Libraries/LibUnicode",
+  ]
+}