Simpleperf: Fix unused parameter

Fix mac build. Follow-up to 638c558339b7f1ae0ed95f64bcf7dbc5b238ed83.

Change-Id: I3065f2ac81db9992f9d4dba2d863402e0a600ee7
diff --git a/simpleperf/environment_fake.cpp b/simpleperf/environment_fake.cpp
index 015c72a..fdcf814 100644
--- a/simpleperf/environment_fake.cpp
+++ b/simpleperf/environment_fake.cpp
@@ -21,6 +21,6 @@
   return false;
 }
 
-bool GetKernelBuildId(BuildId* build_id) {
+bool GetKernelBuildId(BuildId*) {
   return false;
 }