db_test: fix *FLAGS

The -I includes should be in CFLAGS, not CPPFLAGS.

I noticed problems with it when trying to add an -I$(builddir) in an upcoming
patch that adds a BUILT_SOURCE, If77dbbfe1af3e66aaec91cb6295b687f37678636.

Change-Id: Ie57a04b7efc7a1e16cf0e3625d8ad2f0ef0089b0
1 file changed