Fix CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF warnings

When compiling with CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF enabled, the following warnings occur:

    fmdb/src/fmdb/FMDatabaseQueue.m:112:10: Block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior
    fmdb/src/fmdb/FMDatabaseQueue.m:114:9: Block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior

The attached pull request changes the implicit self reference to an explicit self reference to prevent this warning from occurring.