x86, intel-mid: Cleanup some platform code's header files

platform_ipc.h and platform_msic.h are wrongly declaring functions as
external and with 'weak' attribute. This patch does a cleanup on those
header files.

It should have no functional change.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Link: http://lkml.kernel.org/r/1390950567-12821-1-git-send-email-david.a.cohen@linux.intel.com
Cc: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>