Fix Mbed-TLS build when WIN32_LEAN_AND_MEAN macro is defined globally

Signed-off-by: Sergey Markelov <sergey@solidstatenetworks.com>
diff --git a/tf-psa-crypto/drivers/builtin/src/sha256.c b/tf-psa-crypto/drivers/builtin/src/sha256.c
index 49d98c3..842b892 100644
--- a/tf-psa-crypto/drivers/builtin/src/sha256.c
+++ b/tf-psa-crypto/drivers/builtin/src/sha256.c
@@ -152,7 +152,9 @@
     return 1;
 }
 #elif defined(MBEDTLS_PLATFORM_IS_WINDOWS_ON_ARM64)
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#endif
 #include <Windows.h>
 #include <processthreadsapi.h>