perf build: Add a LIBPFM4=1 build test entry

So that when one runs:

$ make -C tools/perf build-test

We make sure that recent changes don't break that opt-in build.

Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Alexey Budankov <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
Cc: Florian Fainelli <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Igor Lubashev <[e-mail]>
Cc: Jin Yao <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Jiwei Sun <[e-mail]>
Cc: John Garry <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Leo Yan <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Martin KaFai Lau <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Yonghong Song <[e-mail]>
Cc: yuzhoujian <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/make

···
 make_no_sdt	    := NO_SDT=1
 make_no_syscall_tbl := NO_SYSCALL_TABLE=1
 make_with_clangllvm := LIBCLANGLLVM=1
+make_with_libpfm4   := LIBPFM4=1
 make_tags           := tags
 make_cscope         := cscope
 make_help           := help
···
 run += make_no_syscall_tbl
 run += make_with_babeltrace
 run += make_with_clangllvm
+run += make_with_libpfm4
 run += make_help
 run += make_doc
 run += make_perf_o