Bluetooth: Mesh: Enforce proper compilation.

The current method relies heavily on the linker/compiler to
do the correct operation. Which is to eliminate the code that will
never get called. This posses a problem if the build even changes
by a smallest fraction.
The current patch will enforce proper inclusion of the code at the
pre-processing stage. Thereby not relying on the compiler/linker to
do the right thing.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>