Fix initialization from incompatible pointer type compile warnings

[ShaneHarvey]

We should fix these compile warnings:

/bsonjs.c -o build/temp.linux-x86_64-3.10/src/bsonjs.o
src/bsonjs.c:234:14: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
     {"dump", dump, METH_VARARGS | METH_KEYWORDS, dump__doc__},
              ^~~~
src/bsonjs.c:234:14: note: (near initialization for ‘BsonjsClientMethods[0].ml_meth’)
src/bsonjs.c:235:15: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
     {"dumps", dumps, METH_VARARGS | METH_KEYWORDS, dumps__doc__},
               ^~~~~