cmake: Only enable CXX when running the coverage

Commit 25a678190c introduced code coverage
collection. That also introduced a dependency to CXX language.

When cross-compiling libssh in an environment that doesn't have a C++ compiler
the following error is raised: "No CMAKE_CXX_COMPILER could be found.".

Since the C++ part is only needed for the coverage part, this commit only enable
that language dependency when actually needing it.

Signed-off-by: Thomas Perale <thomas.perale@mind.be>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

cmake/Modules/AddCMockaTest.cmake

@@ -117,6 +117,7 @@ function(ADD_CMOCKA_TEST _TARGET_NAME)
         ${TARGET_SYSTEM_EMULATOR} ${_TARGET_NAME}
     )
     if (WITH_COVERAGE)
+        ENABLE_LANGUAGE(CXX)
         include(CodeCoverage)
         append_coverage_compiler_flags_to_target(${_TARGET_NAME})
     endif (WITH_COVERAGE)