ext4_utils: move superblock parsing to its own compilation unit

Move ext4_parse_sb to its own compilation unit so it can be used
from fs_mgr without pulling in all of libext4_utils' dependencies
when compiling with ld.bfd.

Change-Id: I185352ecea0e0d577b8cdddfd519f2826d631277
diff --git a/ext4_utils/Android.mk b/ext4_utils/Android.mk
index 6eba3b1..8cb04eb 100644
--- a/ext4_utils/Android.mk
+++ b/ext4_utils/Android.mk
@@ -13,7 +13,8 @@
     uuid.c \
     sha1.c \
     wipe.c \
-    crc16.c
+    crc16.c \
+    ext4_sb.c
 
 #
 # -- All host/targets including windows