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/include/usb_mass_storage.h b/include/usb_mass_storage.h
index 35cdcc3..e08deb4 100644
--- a/include/usb_mass_storage.h
+++ b/include/usb_mass_storage.h
@@ -11,6 +11,7 @@
 #define SECTOR_SIZE		0x200
 
 #include <mmc.h>
+#include <linux/usb/composite.h>
 
 struct ums_device {
 	struct mmc *mmc;
@@ -39,4 +40,12 @@
 extern int usb_gadget_handle_interrupts(void);
 extern int fsg_main_thread(void *);
 
+#ifdef CONFIG_USB_GADGET_MASS_STORAGE
+int fsg_add(struct usb_configuration *c);
+#else
+int fsg_add(struct usb_configuration *c)
+{
+	return 0;
+}
+#endif
 #endif /* __USB_MASS_STORAGE_H__ */