ANDROID: mm: cma: Add forward definition of cma in vendor hook

When compiling usermode linux, following error comes:

include/trace/hooks/mm.h:29:35: warning: 'struct cma' declared inside
parameter list

Add forward declaration of struct cma in trace/hooks/mm.h

Bug: 184732692
Fixes: commit c6e85ea5 ("ANDROID: mm: cma: add vendor hoook in
cma_alloc()")
Change-Id: I650a5b2d7f96a4ecb2d338e3b46abed5ea98459f
Signed-off-by: Elliot Berman <eberman@codeaurora.org>