r600: include libelf headers only as needed

r600: include libelf headers only as needed

Headers are required only when building with OpenCL. As we're building
w/o it libelf may be missing, hence we'll error out as below:

src/gallium/drivers/r600/evergreen_compute.c:27:10:
fatal error: 'gelf.h' file not found
         ^
1 error generated.

Fixes: d96a210842 ("r600g,compute: provide local copy of functions from
ac_binary.c")
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
Reported-by: Mauro Rossi <issor.oruam@gmail.com>
Tested-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>