.gitignore: Add .su files

The '-fstack-usage' option to gcc will generate .su files, ignore them.

Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Tom Rini <trini@ti.com>

.gitignore

@@ -10,6 +10,7 @@
 *.orig
 *.a
 *.o
+*.su
 *~
 *.swp
 *.patch