Makefile: clean lib/efi_loader/helloworld_efi.S

lib/efi_loader/helloworld_efi.S is a generated file and shall be removed by
'make clean'.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/Makefile b/Makefile
index 8b030f3..444baae 100644
--- a/Makefile
+++ b/Makefile
@@ -2160,7 +2160,7 @@
 	       mkimage-out.spl.mkimage mkimage.spl.mkimage imx-boot.map \
 	       itb.fit.fit itb.fit.itb itb.map spl.map mkimage-out.rom.mkimage \
 	       mkimage.rom.mkimage rom.map simple-bin.map simple-bin-spi.map \
-	       idbloader-spi.img
+	       idbloader-spi.img lib/efi_loader/helloworld_efi.S
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config include/generated spl tpl \