kbuild: clang: disable unused variable warnings only when constant

Currently, GCC disables -Wunused-const-variable, but not
-Wunused-variable, so warns unused variables if they are
non-constant.

While, Clang does not warn unused variables at all regardless of
the const qualifier because -Wno-unused-const-variable is implied
by the stronger option -Wno-unused-variable.

Disable -Wunused-const-variable instead of -Wunused-variable so that
GCC and Clang work in the same way.

Change-Id: I2e552288d4a9e19a15fd5e6f4d842e9304e69e28
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>