Support windows static build

Following commit 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.
Merge branch 'support-windows-static-build'

* support-windows-static-build:
  Support windows static build