i2ctinyusb: do not list i2ctinyusb.c twice in SRCS

The module Makefile named its only source file in both the SRCS= line
and the SRCS+= line that follows.  bsd.kmod.mk tolerates the duplicate
in a normal build, but when the kernel is built with -flto=thin the
linker sees the same bitcode object twice and refuses to link the
module ("Expected at most one ThinLTO module per bitcode file").

Fixes:		54e231b373
MFC after:	1 week

sys/modules/usb/i2ctinyusb/Makefile

@@ -29,7 +29,6 @@ S=     ${SRCTOP}/sys
 .PATH: $S/dev/usb/misc
 
 KMOD=	i2ctinyusb
-SRCS=	i2ctinyusb.c
-SRCS+=	i2ctinyusb.c device_if.h bus_if.h opt_usb.h usbdevs.h iicbus_if.h
+SRCS=	i2ctinyusb.c device_if.h bus_if.h opt_usb.h usbdevs.h iicbus_if.h
 
 .include <bsd.kmod.mk>