tests/checkasm/checkasm: Do not define an unused function.

Fixes the following warning:
tests/checkasm/checkasm.c:615:12: warning: 'bench_init_ffmpeg' defined but not used

tests/checkasm/checkasm.c

@@ -612,6 +612,7 @@ static int bench_init_linux(void)
 }
 #endif
 
+#if !CONFIG_LINUX_PERF
 static int bench_init_ffmpeg(void)
 {
 #ifdef AV_READ_TIME
@@ -622,6 +623,7 @@ static int bench_init_ffmpeg(void)
     return -1;
 #endif
 }
+#endif
 
 static int bench_init(void)
 {