Add test program for C++ support

So that we don't regress

examples/cpp-test.cpp

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include <libfprint/fprint.h>
+
+int main (int argc, char **argv)
+{
+	fp_init ();
+	return 0;
+}