.gitignore: add *.su pattern

Ignore the *.su files generated by using the gcc option -fstack-usage.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Michal Marek <mmarek@suse.com>

.gitignore

@@ -36,6 +36,7 @@
 modules.builtin
 Module.symvers
 *.dwo
+*.su
 
 #
 # Top-level generic files