.gitignore: ignore *.cover and *.mbx

The 'b4' command creates a *.mbx file, and also a *.cover file if the
patch set has a cover-letter. Ignore them.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

.gitignore

@@ -16,6 +16,7 @@
 *.bin
 *.bz2
 *.c.[012]*.*
+*.cover
 *.dt.yaml
 *.dtb
 *.dtbo
@@ -33,6 +34,7 @@
 *.lz4
 *.lzma
 *.lzo
+*.mbx
 *.mod
 *.mod.c
 *.o