Let the dynamic linker try to resolve wrapper preload if apitrace
has been installed system-wide.

If apitrace has been installed system-wide, presumably the wrappers
have been installed in a subdirectory of somewhere that ld.so can find.

Delegating to ld.so with ${LIB} allows users to install both 32-bit
and 64-bit wrappers and apitrace can use whichever is appropriate.

Signed-off-by: Christopher James Halse Rogers <raof@ubuntu.com>