trace/events: fix compilation error

include/trace/events/filemap.h: In function 'ftrace_raw_output_mm_filemap_find_page_cache_miss':
include/trace/ftrace.h:232:9: error: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'size_t' [-Werror=format=]

Change-Id: Ic2d76f18fc8802cf1e2246f96d84a06d267a30ad
Signed-off-by: Jin Qian <jinqian@google.com>