drivers: edac: build as static library

Instead of putting object files inside libzephyr.a,
simply build a separate static library as most other
driver types are doing this already.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
diff --git a/drivers/edac/CMakeLists.txt b/drivers/edac/CMakeLists.txt
index a32e820..52f6f00 100644
--- a/drivers/edac/CMakeLists.txt
+++ b/drivers/edac/CMakeLists.txt
@@ -1,5 +1,7 @@
 # Copyright (c) 2020 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
 
-zephyr_sources_ifdef(CONFIG_EDAC_IBECC edac_ibecc.c)
-zephyr_sources_ifdef(CONFIG_EDAC_SHELL shell.c)
+zephyr_library()
+
+zephyr_library_sources_ifdef(CONFIG_EDAC_IBECC edac_ibecc.c)
+zephyr_library_sources_ifdef(CONFIG_EDAC_SHELL shell.c)