Add a missing include to the Linux plugin template (#68737)

The template uses strcmp (to check the received method), so should
include <cstring>

packages/flutter_tools/templates/plugin/linux.tmpl/pluginClassSnakeCase.cc.tmpl

@@ -4,6 +4,8 @@
 #include <gtk/gtk.h>
 #include <sys/utsname.h>
 
+#include <cstring>
+
 #define {{pluginClassCapitalSnakeCase}}(obj) \
   (G_TYPE_CHECK_INSTANCE_CAST((obj), {{pluginClassSnakeCase}}_get_type(), \
                               {{pluginClass}}))