msm: wfd: Explicitly include slab.h

slab.h is getting implicitly included from external header files, which
if disabled in compilation, causes compilation errors for wfd driver.
Include slab.h explicitly to remove this unnecessary dependancy.

Change-Id: I23691038f92ad1f6b9f2225a39ea76d0a18573ab
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>