add C implementation for mandelbrot in the yaml

Author: ElekCorp

bench/bench_c.yaml

@@ -20,6 +20,9 @@ problems:
       - 4.c
       - 5-im.c
       - 6-im.c
+  - name: mandelbrot
+    source:
+      - 1-ffi.c
   - name: nsieve
     source:
       - 1.c