CMake script refers to obsolete path

[jphickey]

Describe the bug
The CMake build script for elf2cfetbl is outdated and refers to a CFE path that no longer exist in the CFE bundle.

elf2cfetbl/CMakeLists.txt

Line 5 in b6aa1c0

include_directories(${cfe-core_MISSION_DIR}/src/inc)

Expected behavior
At a minimum, the incorrect path should be removed
A better approach altogether would be to use an interface library that refers to the OSAL API

Additional context
Doesn't really break anything because nonexistent paths are typically ignored by GCC (although that may not be the case with some other compilers)

Reporter Info
Joseph Hickey, Vantage Systems, Inc.