py: Separate MICROPY_PY_BUILTINS_COMPLEX from MICROPY_PY_BUILTINS_FLOAT.

One thing is wanting to do 1 / 2 and get something else but 0, and quite
another - doing rocket science ;-).
diff --git a/py/builtin.c b/py/builtin.c
index b25ca42..d4b77d3 100644
--- a/py/builtin.c
+++ b/py/builtin.c
@@ -113,11 +113,13 @@
         } else {
             return o_in;
         }
+#if MICROPY_PY_BUILTINS_COMPLEX
     } else if (MP_OBJ_IS_TYPE(o_in, &mp_type_complex)) {
         mp_float_t real, imag;
         mp_obj_complex_get(o_in, &real, &imag);
         return mp_obj_new_float(MICROPY_FLOAT_C_FUN(sqrt)(real*real + imag*imag));
 #endif
+#endif
     } else {
         assert(0);
         return mp_const_none;