Makefile: remove generated font files

If `make mrproper` does not delete the generated drivers/video/fonts/*.S
files a following `make tests` fails.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 9ca414e..e8f52b4 100644
--- a/Makefile
+++ b/Makefile
@@ -1740,7 +1740,8 @@
 MRPROPER_DIRS  += include/config include/generated spl tpl \
 		  .tmp_objdiff
 MRPROPER_FILES += .config .config.old include/autoconf.mk* include/config.h \
-		  ctags etags tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
+		  ctags etags tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS \
+		  drivers/video/fonts/*.S
 
 # clean - Delete most, but leave enough to build external modules
 #