tracing: tell git to ignore the generated 'undefsyms_base.c' file

This odd file was added to automatically figure out tool-generated
symbols.

Honestly, it *should* have been just a real honest-to-goodness regular
file in git, instead of having strange code to generate it in the
Makefile, but that is not how that silly thing works.  So now we need to
ignore it explicitly.

Fixes: 1211907a ("tracing: Generate undef symbols allowlist for simple_ring_buffer")
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>