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)
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index c58475f..68b42df 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -261,8 +261,7 @@
 endef
 
 ifeq ($(HOST_OS),darwin)
-  MAKE_FLAGS += C_INCLUDE_PATH=$(ANDROID_BUILD_TOP)/external/elfutils/libelf/
-  MAKE_FLAGS += C_INCLUDE_PATH=/usr/local/opt/openssl/include
+  MAKE_FLAGS += C_INCLUDE_PATH=$(ANDROID_BUILD_TOP)/external/elfutils/libelf:/usr/local/opt/openssl/include
   MAKE_FLAGS += LIBRARY_PATH=/usr/local/opt/openssl/lib
 endif