icE1usb fw: Add missing usb_e1.h in the Makefile header list

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: I2088700c0fc99dbb2940956dd9066ea098ef3eb6
diff --git a/firmware/ice40-riscv/icE1usb/Makefile b/firmware/ice40-riscv/icE1usb/Makefile
index b2f85d1..06f5484 100644
--- a/firmware/ice40-riscv/icE1usb/Makefile
+++ b/firmware/ice40-riscv/icE1usb/Makefile
@@ -49,6 +49,7 @@
 	e1.h \
 	misc.h \
 	usb_desc_ids.h \
+	usb_e1.h \
 	usb_str_app.gen.h \
 	$(NULL)