perf tools: Don't hardcode host include path for libslang

Hardcoding /usr/include/slang is fundamentally incompatible with cross
compilation and will lead to the inability for a cross-compiled
environment to properly detect whether slang is available or not.

If /usr/include/slang is necessary that is a distribution specific
knowledge that could be solved with either a standard pkg-config .pc
file (which slang has) or simply overriding CFLAGS accordingly, but the
default perf Makefile should be clean of all of that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Monnet <quentin.monnet@netronome.com>
Cc: Stanislav Fomichev <sdf@google.com>
Fixes: ef7b93a1 ("perf report: Librarize the annotation code and use it in the newt browser")
Link: http://lkml.kernel.org/r/20190614183949.5588-1-f.fainelli@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>