msm: kgsl: Remove stddef.h include

Having this include here causes sparse to spew an error about
including a file outside of the kerenel repo. There's a
linux/stddef.h file that could be included instead, but things
compile without this include anyway so just remove it.

Change-Id: I01eaef20d6cf5078dece73347b61803ca46942bb
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>