xtensa: tell git to ignore copied zlib source files

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/boot/lib/.gitignore

@@ -0,0 +1,3 @@
+inffast.c
+inflate.c
+inftrees.c