Fix psa-iat-api-test build error

When a pre-built MbedTLS binary is used with the psa-iat-api-test, the
build fails with:
    Couldn't find psa/crypto.h

This is because MbedTLS.cmake is only defining PSA_CRYPTO_API_INCLUDE
when the source-code is fetched. This variable is used later to set an
include path which allows the compiler to find psa/crypto.h.
This change moves setting the variable to a ensure it is set always.

Change-Id: I63b9f796992ce03514438647ff8726026ce42cdc
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/external/MbedTLS/MbedTLS.cmake b/external/MbedTLS/MbedTLS.cmake
index 3193a07..78efdb7 100644
--- a/external/MbedTLS/MbedTLS.cmake
+++ b/external/MbedTLS/MbedTLS.cmake
@@ -96,9 +96,6 @@
 	#Configure Mbed TLS to build only mbedcrypto lib
 	execute_process(COMMAND ${Python3_EXECUTABLE} scripts/config.py crypto WORKING_DIRECTORY ${MBEDTLS_SOURCE_DIR})
 
-	# Advertise Mbed TLS as the provider of the psa crypto API
-	set(PSA_CRYPTO_API_INCLUDE "${MBEDTLS_INSTALL_DIR}/include" CACHE STRING "PSA Crypto API include path")
-
 	include(${TS_ROOT}/tools/cmake/common/PropertyCopy.cmake)
 
 	# Only pass libc settings to MbedTLS if needed. For environments where the standard
@@ -157,6 +154,9 @@
 	set(MBEDCRYPTO_LIB_FILE "${MBEDTLS_INSTALL_DIR}/lib/${CMAKE_STATIC_LIBRARY_PREFIX}mbedcrypto${CMAKE_STATIC_LIBRARY_SUFFIX}")
 endif()
 
+# Advertise Mbed TLS as the provider of the psa crypto API
+set(PSA_CRYPTO_API_INCLUDE "${MBEDTLS_INSTALL_DIR}/include" CACHE STRING "PSA Crypto API include path")
+
 #Create an imported target to have clean abstraction in the build-system.
 add_library(mbedcrypto STATIC IMPORTED)
 set_property(DIRECTORY ${CMAKE_SOURCE_DIR} APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${MBEDCRYPTO_LIB_FILE})