"Include what you use" for tests

Using strcpy/strlen in global namespace requires `<string.h>`. Include
explicitly instead of assuming another SWIG or library header brings it
in.

Examples/test-suite/kwargs_feature.i

@@ -128,6 +128,11 @@ struct ExtendingOptArgs1 {};
 struct ExtendingOptArgs2 {};
 %}
 
+// For strlen/strcpy
+%{
+#include <string.h>
+%}
+
 // Varargs
 %warnfilter(SWIGWARN_LANG_VARARGS_KEYWORD) VarargConstructor::VarargConstructor; // Can't wrap varargs with keyword arguments enabled
 %warnfilter(SWIGWARN_LANG_VARARGS_KEYWORD) VarargConstructor::vararg_method; // Can't wrap varargs with keyword arguments enabled