Export sdl mock classes

#251

tests/src/mocks/sdl_mock.hpp

@@ -31,7 +31,7 @@ SUPPRESS_WARNINGS_END
 
 #include "trompeloeil.hpp"
 
-class SdlMockInternal {
+class SDL_MOCK_EXPORT SdlMockInternal {
  public:
   MAKE_MOCK1(SDL_Init, int(Uint32 flags));  // NOLINT(readability/casting)
   MAKE_MOCK1(SDL_InitSubSystem,
@@ -55,7 +55,7 @@ class SdlMockInternal {
   MAKE_MOCK1(SDL_HideWindow, void(SDL_Window*));
 };
 
-class SdlMock {
+class SDL_MOCK_EXPORT SdlMock {
  public:
   SdlMock() : mock_(new SdlMockInternal) {}
   SdlMock(const SdlMock&) = delete;