kernel: Fix broken C_INCLUDE_PATH for Darwin

Commit 7fb9251dfcc7a0876c8fb906338452204942144a modified
the expansion in an illegal way.

Fix the broken env variable.

Change-Id: Ib3029fa995d6a4b0416887a2ef2e4792c9e6bb27
(cherry picked from commit 90b437e784c19fc428c24891cc96705b89b786df)
1 file changed