BUG: Add missing export specifiers for MRC module

The IOMRC module was specified as a export controlled shared library
but no exports were specified.This resulted in link errors only for
the __Private method. The correct header and macros are now added.

Change-Id: Iaafe5215f524b01a6854dce60ff82049ba4241ea
ENH: Move MRC wrapping out of review

Change-Id: Ib187c085b6b17754c6b01355e764173be628c208
Merge topic 'FixMRCImageIOExport'

9bcb077 ENH: Move MRC wrapping out of review
01312e8 BUG: Add missing export specifiers for MRC module