cmake: Fix capitalization of CSP_ZEPHYR
cmake: Fix capitalization of CSP_ZEPHYR

This is a typo.  It should be CSP_ZEPHYR instead.  CSP_ZEPHYR is
already set to 1 at the top CMakeLists.txt so it doesn't have any
effect.  It is here to imitate meson.build.

Signed-off-by: Yasushi SHOJI <afa9ca3be149447780c2f7b524d61ed3d3db63f9@spacecubics.com>

libcsp/libcsp,libcsp/libcsp