Fix clang warnings about inconsistent use of dllexport

Fix clang warnings about inconsistent use of dllexport

In MSYS2, clang emits many warnings like the following:

    warning: redeclaration of 'wimlib_add_empty_image' should not add 'dllexport' attribute

The problem is that dllexport is used when defining the wimlib API
functions but not when declaring them.  Fix this by using dllexport in
both places.