perf test: Add llvm-src-base.c and llvm-src-kbuild.c to .gitignore

Commit b31de018 ("perf test: Enhance the
LLVM test: update basic BPF test program") dynamically creates file
llvm-src-base.c during the perf building.

Similarly, the commit 7af3f3d5 ("perf
test: Enhance the LLVM tests: add kbuild test") dynamically creates file
llvm-src-kbuild.c during the perf building. Add them to .gitignore.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: b31de018 ("perf test: Enhance the LLVM test: update basic BPF test program")
Fixes: 7af3f3d5 ("perf test: Enhance the LLVM tests: add kbuild test")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>