ANDROID: fix build error in trace_events_synth.c

The android build makes all warnings errors, so the warning added by
commit 72848b81 ("tracing: Ensure module defining synth event cannot
be unloaded while tracing") needs to be fixed up in order to keep the
build working properly.

Fixes: 72848b81 ("tracing: Ensure module defining synth event cannot be unloaded while tracing")
Change-Id: I3b5cbce5f83a9405e1b057d0e6bb64d53c25d131
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>