[sdk][build] Only reference header files once in build rule

Use `public` directly rather than setting its value to `sources`. This
is correct and avoids an error about a file being defined by multiple
atoms (though it is just this one) during IDK generation when building
//sdk/firmware.

Change-Id: I0e5722952f6c8a98e714d1fc3d61bb5df59bc25e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/vboot_reference/+/1357835
Reviewed-by: Yecheng Zhao <zyecheng@google.com>