Set CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS (#889)
Set CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS (#889)

When building on Windows with `BUILD_SHARED_LIBS=ON`, the symbols were
not being properly exported in the DLL. Fix this by setting
`CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS`.

Fixes #888 
google/benchmark,google/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark,google/benchmark,mockingbirdnest/benchmark