[url_launcher_linux] Add missing #include (#3184)

The plugin code uses strcmp, so should include cstring. Currently this
works only due to transitive includes that we can't count on being
present.
diff --git a/packages/url_launcher/url_launcher_linux/CHANGELOG.md b/packages/url_launcher/url_launcher_linux/CHANGELOG.md
index 7855c4a..c6a0042 100644
--- a/packages/url_launcher/url_launcher_linux/CHANGELOG.md
+++ b/packages/url_launcher/url_launcher_linux/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.0.1+3
+
+* Add a missing include.
+
 ## 0.0.1+2
 
 * Check in linux/ directory for example/
diff --git a/packages/url_launcher/url_launcher_linux/linux/url_launcher_plugin.cc b/packages/url_launcher/url_launcher_linux/linux/url_launcher_plugin.cc
index 592bb96..8ef1a8a 100644
--- a/packages/url_launcher/url_launcher_linux/linux/url_launcher_plugin.cc
+++ b/packages/url_launcher/url_launcher_linux/linux/url_launcher_plugin.cc
@@ -7,6 +7,8 @@
 #include <flutter_linux/flutter_linux.h>
 #include <gtk/gtk.h>
 
+#include <cstring>
+
 // See url_launcher_channel.dart for documentation.
 const char kChannelName[] = "plugins.flutter.io/url_launcher";
 const char kBadArgumentsError[] = "Bad Arguments";
diff --git a/packages/url_launcher/url_launcher_linux/pubspec.yaml b/packages/url_launcher/url_launcher_linux/pubspec.yaml
index 2e3a1ba..c4f6f8c 100644
--- a/packages/url_launcher/url_launcher_linux/pubspec.yaml
+++ b/packages/url_launcher/url_launcher_linux/pubspec.yaml
@@ -1,6 +1,6 @@
 name: url_launcher_linux
 description: Linux implementation of the url_launcher plugin.
-version: 0.0.1+2
+version: 0.0.1+3
 homepage: https://github.com/flutter/plugins/tree/master/packages/url_launcher/url_launcher_linux
 
 flutter:
@@ -16,4 +16,3 @@
 dependencies:
   flutter:
     sdk: flutter
-