diag: Only include MHI headers if it is enabled

Only include the MHI header file if CONFIG_MSM_MHI
is enabled, avoid compilation errors if the platform
does not support MHI.

Change-Id: Ic2d84a8bbd066d0d8e50711a7499ae9a959a0b71
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>