X.org: needs dynamic library support

xlib_libX11, one of the main libraries of X.org, uses dlfcn.h and dlopen.
To avoid having to add a dependency on !BR2_PREFER_STATIC_LIB on tons of
packages, we promote this dependency to X.org itself (similar to how thread
support is handled in X.org).

Fixes
http://autobuild.buildroot.net/results/a47/a47c5d96f880af04b85bb6773e87d04f9d7c2864/



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>