Add/remove includes to match used types.

uint64_t and similar types are defined in stdint.h, and size_t is
defined in stddef.h. While these headers are included virtually
everywhere anyways we should include what we use, specially in public
headers.

Bug: None
Test: still compiles.

Change-Id: I26846be960bf9c989372a05e9c4369caf02888cd