COMP: Fix inconsistent linkage error related to qRestAPI

Associated qRestAPI commit is commontk/qRestAPI@6339e0c and
the corresponding message as been copied below:

// -------------
Support windows static build

Following commit commontk/qRestAPI@5c056f2, the project could be built as a shared library
this caused problem on windows where the qRestAPI_EXPORT macro was
also defined in the case of static build causing an "inconsistent linkage"
error.

This commit explicitly define qRestAPI_EXPORT based on the
BUILD_SHARED_LIBS option.
// -------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22030 3bd1e089-480b-0410-8dfb-8563597acbee