xtest: add -DOPENSSL_API_COMPAT=10100

regression_1000.c uses OpenSSL functions that have been deprecated
after version 1.1. In particular, building with version 3.0 causes the
following error (and other similar ones not pasted here):

 optee_test/host/xtest/regression_1000.c: In function 'check_signature':
 optee_test/host/xtest/regression_1000.c:2735:9: error: 'SHA256_Init' is deprecated: Since OpenSSL 3.0 [-Werror=deprecated-declarations]
  2735 |         SHA256_Init(&ctx);
       |         ^~~~~~~~~~~

Add -DOPENSSL_API_COMPAT=10100 to the build flags to avoid this and be
flexible with regards to which version of OpenSSL build environment has
to provide.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/host/xtest/Makefile b/host/xtest/Makefile
index ce55c3f..dbfd392 100644
--- a/host/xtest/Makefile
+++ b/host/xtest/Makefile
@@ -39,7 +39,7 @@
 # - User/group login tests 1027 and 1028
 WITH_OPENSSL ?= y
 ifeq ($(WITH_OPENSSL),y)
-CFLAGS += -DOPENSSL_FOUND=1
+CFLAGS += -DOPENSSL_FOUND=1 -DOPENSSL_API_COMPAT=10100
 ifneq ($(OPTEE_OPENSSL_EXPORT),)
 LDFLAGS += -lcrypto
 CFLAGS += -I$(OPTEE_OPENSSL_EXPORT)