rtla: Fix and clean up .gitignore

rtla: Fix and clean up .gitignore

.gitignore includes several entries prone to unwanted matches in
subdirectories. One of them, the recently added "lib/", matches the
recently added directory "tests/scripts/lib/" in addition to the
intended top-level "lib/", which contains object files built from
sources in tools/lib.

Add "/" to all .gitignore entries that are intended to only match
top-level files or directories: rtla, rtla_static, unit_tests,
libsubcmd/.

Remove .gitignore entries that are not needed at all:

- lib/ (contains only object files, ignored by top-level .gitignore
  already).
- .txt rtla output files added to .gitignore in commit 02689ae385c5
  ("rtla: Add generated output files to gitignore"). Since commit
  ad5b50a0959f ("rtla/tests: Run runtime tests in temporary directory"),
  those are created in a temporary directory, not in tools/tracing/rtla.

Keeping libsubcmd/ as that contains other generated files (headers,
archives, etc.).

Fixes: 48209d763c22 ("rtla: Add libsubcmd dependency")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202605291919.eszupseg-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202605300436.PqQ0Bc8q-lkp@intel.com/
Link: https://lore.kernel.org/r/20260601091835.3118094-1-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>