services: TRNG: Fix -O0 compilation

The code to check for the presence of the TRNG service relies on
toolchain garbage collection, which is not enabled with -O0.

Add #ifdef guards around the call to the TRNG service handler to
cover builds without optimisation as well.

Change-Id: I08ece2005ea1c8fa96afa13904a851dec6b24216
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

services/std_svc/std_svc_setup.c

@@ -142,10 +142,12 @@ static uintptr_t std_svc_smc_handler(uint32_t smc_fid,
 	}
 #endif
 
+#if TRNG_SUPPORT
 	if (is_trng_fid(smc_fid)) {
 		return trng_smc_handler(smc_fid, x1, x2, x3, x4, cookie, handle,
 				flags);
 	}
+#endif
 
 	switch (smc_fid) {
 	case ARM_STD_SVC_CALL_COUNT: