sdl_net: use sdl_net instead of SDL_net as make target

As suggested by Julien Boibessot <julien.boibessot@free.fr>.

The other SDL extensions use sdl_xx, and not SDL_xx, so rename sdl_net
to match.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/sdl_net/sdl_net.mk

@@ -14,4 +14,4 @@ SDL_NET_INSTALL_STAGING = YES
 
 SDL_NET_DEPENDENCIES = sdl
 
-$(eval $(call AUTOTARGETS,package,SDL_net))
+$(eval $(call AUTOTARGETS,package,sdl_net))