Only link xbmc/main/main.o when building xbmc.bin

Currently i have to recompile XBMC as a library (with configure option --enable-shared-lib) to be able to compile the gtests. Otherwise it complains about multiple definitions of main().

This PR changes that by not always linking in xbmc/main/main.a, so that xbmc-test can link in its own main() function.