Raise level of -Wformat-truncation to 2

This was not possible earlier due to warnings, however now this seems to
be ok.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c56fb77..2ab2e01 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -198,8 +198,7 @@
         endif()
     endif()
     if (GCC_VERSION VERSION_GREATER 7.0 OR GCC_VERSION VERSION_EQUAL 7.0)
-      # Cannot use -Wformat-truncation=2 as this fails on mbedtls_snprintf (unknown buffer size)
-      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat-overflow=2 -Wformat-truncation")
+      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat-overflow=2 -Wformat-truncation=2")
     endif()
     set(CMAKE_C_FLAGS_RELEASE     "-O2")
     set(CMAKE_C_FLAGS_DEBUG       "-O0 -g3")