qemu: Add "-Wno-error" flag to qemu target

GCC 5 and above fails to build OP-TEE project under QEMU.
Warnings are now treated as errors by default.

On line 60 --extra-cflags="-Wno-error" fixes the issue.
Please reference
[OP-TEE/optee_os#550](https://github.com/OP-TEE/optee_os/issues/550) for
specifics.

My machine is running gcc version 5.2.1 20151010 and I can confirm this
now works with change. It affects both default.xml and default_stable.xml repo
configurations for OP-TEE.

Reviewed-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jorden Whitefield <j.whitefield@surrey.ac.uk>
diff --git a/qemu.mk b/qemu.mk
index 1ae3030..9604ce0 100644
--- a/qemu.mk
+++ b/qemu.mk
@@ -57,7 +57,7 @@
 	$(call bios-qemu-common) clean
 
 qemu:
-	cd $(QEMU_PATH); ./configure --target-list=arm-softmmu --cc="$(CCACHE)gcc"
+	cd $(QEMU_PATH); ./configure --target-list=arm-softmmu --cc="$(CCACHE)gcc" --extra-cflags="-Wno-error"
 	$(MAKE) -C $(QEMU_PATH)
 
 qemu-clean: