testutil/driver.c: Fix function prototype warning [-Wstrict-prototypes]

(introduced by commit 91860165, which added -Wstrict-prototypes)
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6555)

test/testutil/driver.c

@@ -74,7 +74,7 @@ int subtest_level(void)
 }
 
 #ifndef OPENSSL_NO_CRYPTO_MDEBUG
-static int should_report_leaks()
+static int should_report_leaks(void)
 {
     /*
      * When compiled with enable-crypto-mdebug, OPENSSL_DEBUG_MEMORY=0