perf build: Fix unescaped # in perf build-test

With the following bash and make versions:

  $ make --version
  GNU Make 4.2.1
  Built for aarch64-unknown-linux-gnu

  $ bash --version
  GNU bash, version 5.0.17(1)-release (aarch64-unknown-linux-gnu)

This error is encountered when running the build-test target:

  $ make -C tools/perf build-test
  tests/make:181: *** unterminated call to function 'shell': missing ')'.  Stop.
  make: *** [Makefile:103: build-test] Error 2

Fix it by escaping the # which was causing make to interpret the rest of
the line as a comment leaving the unclosed opening bracket.

Fixes: 56d52294 ("tools build: Pass libbpf feature only if libbpf 1.0+")
Signed-off-by: James Clark <james.clark@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20230425104414.1723571-1-james.clark@arm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/make

@@ -178,7 +178,7 @@ run += make_install_prefix_slash
 # run += make_install_pdf
 run += make_minimal
 
-old_libbpf := $(shell echo "#include <bpf/libbpf.h>" | $(CC) -E -dM -x c -| egrep -q "define[[:space:]]+LIBBPF_MAJOR_VERSION[[:space:]]+0{1}")
+old_libbpf := $(shell echo '\#include <bpf/libbpf.h>' | $(CC) -E -dM -x c -| egrep -q "define[[:space:]]+LIBBPF_MAJOR_VERSION[[:space:]]+0{1}")
 
 ifneq ($(old_libbpf),)
 run += make_libbpf_dynamic