build: Remove use of BUILT_SOURCES from Makefile.win32

Since 3d81d89c BUILT_SOURCES is not
used anymore, but it was unintentionally left in Win32 Makefiles.

Signed-off-by: Andrea Canciani <ranma42@gmail.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>