alsamixergui: depend on x11 support, rather than (indirectly) selecting it

Packages should depend on BR2_PACKAGE_XORG7 rather than selecting it,
because:
 - X brings in a lot of dependencies, so shouldn't get implicitly enabled
 - Select doesn't check dependencies, so build breaks if XORG dependencies
   aren't met (locale/wchar)

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