xtest: fix 32bit/64bit build directive when GP suite is enabled

OP-TEE core configuration directive for AArch32 is CFG_ARM32_core not
CFG_ARM32.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/host/xtest/Makefile b/host/xtest/Makefile
index 1c5f6d6..af9e1f0 100644
--- a/host/xtest/Makefile
+++ b/host/xtest/Makefile
@@ -110,7 +110,7 @@
 CFLAGS += -Ifor_gp/include
 
 # by default, the client application is compiled as the kernel of optee-os
-ifeq ($(CFG_ARM32),y)
+ifeq ($(CFG_ARM32_core),y)
 COMPILE_NS_USER ?= 32
 else
 COMPILE_NS_USER ?= 64