tools build: Drop needless slang include path in test-all

Commit cbefd24f ("tools build: Add test to check if slang.h is
in /usr/include/slang/") added a proper test to check whether slang.h is
in a subdirectory, and commit 1955c8cf

 ("perf tools: Don't
hardcode host include path for libslang") removed the include path for
test-libslang.bin but missed test-all.bin.

Apply the same change to test-all.bin.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Fixes: 1955c8cf

 ("perf tools: Don't hardcode host include path for libslang")
Signed-off-by: John Keeping <john@metanate.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20211025172314.3766032-1-john@metanate.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>