Forwarding tests not installed even if GKFS_INSTALL_TESTS is ON

Forwarding tests are not properly installed even if GKFS_BUILD_TESTS and GKFS_INSTALL_TESTS are both ON. The reason is a small typo in CMakeLists.txt where ENABLE_FORWARDING should actually be GKFS_ENABLE_FORWARDING:

    message(STATUS "[gekkofs] Check for forwarding tests...")
    if (ENABLE_FORWARDING)
        set(GKFS_TESTS_FORWARDING "ON" CACHE STRING "Enable I/O forwarding tests (default: OFF)")
    else ()
        set(GKFS_TESTS_FORWARDING "OFF" CACHE STRING "Enable I/O forwarding tests (default: OFF)")
    endif ()
    message(STATUS "[gekkofs] Forwarding tests: ${GKFS_TESTS_FORWARDING}")