Adjust the format specifier based on the typedef for wg_yajl_integer_t.

[igorpeshansky]

Should fix the following build error on platforms with libyajl1:

src/utils_stackdriver_json.c: In function 'summary_parse_integer':
src/utils_stackdriver_json.c:196:3: error: format '%lld' expects argument of type 'long long int', but argument 3 has type 'wg_yajl_integer_t' [-Werror=format=]
   DEBUG("integer: %lld", val);
   ^