libsdl2: Fix build with static libraries disabled

libsdl2: Fix build with static libraries disabled

libsdl2 was using foo.o in Makefile dependencies but if libtool is
being used then those are the statically linked intermediate files.
 When static libraries are globally disabled they won't be generated by
libtool so the build fails.

Instead, use the libtool intermediate wrapper .lo files so the build
succeeds.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>