diag: Fix compile error due to missing msm_mhi.h

diag_debugfs.c includes diagfwd_mhi.h which includes msm_mhi.h which
has not been ported over yet. Since the MHI dependency is only needed
when CONFIG_DIAGFWD_BRIDGE_CODE is enabled, move the header include
to within the corresponding #ifdef.

Signed-off-by: Jack Pham <jackp@codeaurora.org>