Changed xdebug config setting from size_t to uint64_t

Summary: In issue #3227 compiling failed due to XDebugExtension::FramebufSize having type
size_t because Config::Bind does not have a definition for size_t. We didn't run
into this issue when testing or compiling internally. But the user
compiled on mac osx, where `size_t` expanded to `long unsigned int` and
`uint64_t` expanded to `long long unsigned int`.

This diff changes the configuration option to have type uint64_t, which should
fix the error.

Reviewed By: @ptarjan

Differential Revision: D1452470