usb:g_dnl:ums: Conditional compilation for mass storage function (f_mass_storage)

The mass storage composite function is now compiled in only when
CONFIG_USB_GADGET_MASS_STORAGE is defined.
Such change provides binary size reduction for boards which use USB
download gadget (like am335x_evm) with DFU, but don't use UMS.

For example at am335x_evm board reduction is more than 2KiB for
text and around 120B for data.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>
diff --git a/drivers/usb/gadget/g_dnl.c b/drivers/usb/gadget/g_dnl.c
index a3e05a8..19011bf 100644
--- a/drivers/usb/gadget/g_dnl.c
+++ b/drivers/usb/gadget/g_dnl.c
@@ -15,11 +15,11 @@
 #include <part.h>
 
 #include <g_dnl.h>
+#include <usb_mass_storage.h>
 #include "f_dfu.h"
 
 #include "gadget_chips.h"
 #include "composite.c"
-#include "f_mass_storage.c"
 
 /*
  * One needs to define the following: