perf build: Rename LIB_PATH -> API_PATH

'LIB_PATH' is a misnomer because there are multiple library paths.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/c10df0b749a27f05cc531fe06b8dd71a329341fa.1449965119.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>