gitignore: ignore all build* subdirs in main directory

Now that meson builds require a separate build dir, everybody's bound to have
at least one. So let's accept the convention that build*/ is for builds, and
apply a blanket ignore rule, we shouldn't have any important files named build*
in the main directory.

.gitignore

@@ -28,7 +28,7 @@
 /TAGS
 /ata_id
 /bootctl
-/build-aux
+/build*
 /busctl
 /cdrom_id
 /collect