corrected cmake path

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/programs/fuzz/CMakeLists.txt b/programs/fuzz/CMakeLists.txt
index 5dbc928..61c5b63 100644
--- a/programs/fuzz/CMakeLists.txt
+++ b/programs/fuzz/CMakeLists.txt
@@ -37,7 +37,7 @@
     # This emulates "if ( ... IN_LIST ... )" which becomes available in CMake 3.3
     list(FIND executables_with_common_c ${exe} exe_index)
     if(${exe_index} GREATER -1)
-        list(APPEND exe_sources ../../tf-psa-crypto/programs/fuzz/common.c)
+        list(APPEND exe_sources ${MBEDTLS_DIR}/tf-psa-crypto/programs/fuzz/fuzz_common.c)
     endif()
 
     add_executable(${exe} ${exe_sources})