include: dfu: Add extern "C" in mcuboot.h

I ran into a build failure trying to use Zephyr's MCUmgr.  It was a
missing symbol at link time, and since I am using C++, I looked to see
if it was a name mangling issue.  The mcuboot.h header file was missing
`extern "C"` guards, which was the root cause of the issue.

This commit adds C++ support to mcuboot.h by adding in `extern "C"`
guards.  I validated this change by building and running my DFU
application with MCUmgr successfully.

Signed-off-by: Brooks Prumo <brooks@prumo.org>