VisualC: Set the correct component to false.

When SDL2_main would not be found, it would incorrectly flag the shared SDL2 library as not found.

VisualC/pkg-support/cmake/sdl2-config.cmake

@@ -94,7 +94,7 @@ if(EXISTS "${_sdl2main_library}")
     endif()
     set(SDL2_SDL2main_FOUND TRUE)
 else()
-    set(SDL2_SDL2_FOUND FALSE)
+    set(SDL2_SDL2main_FOUND FALSE)
 endif()
 unset(_sdl2main_library)