testsuite: coverage: fix typo in the CMakeLists.txt

`zephyr_BASE` should have been `ZEPHYR_BASE`, fix it.

Signed-off-by: Yong Cong Sin <[email protected]>
(cherry picked from commit a5555ab)

Author: ycsin

subsys/testsuite/CMakeLists.txt

@@ -6,6 +6,6 @@ if(CONFIG_TEST)
   zephyr_include_directories(${ZEPHYR_BASE}/subsys/testsuite/include)
 endif()
 add_subdirectory_ifdef(CONFIG_COVERAGE_GCOV coverage)
-zephyr_include_directories_ifdef(CONFIG_COVERAGE_GCOV ${zephyr_BASE}/subsys/testsuite/coverage)
+zephyr_include_directories_ifdef(CONFIG_COVERAGE_GCOV ${ZEPHYR_BASE}/subsys/testsuite/coverage)
 
 zephyr_library_sources_ifdef(CONFIG_TEST_BUSY_SIM busy_sim/busy_sim.c)