perf tools: Create pager.h

Move the 'pager' function prototypes into a new pager.h so that the
pager code can be moved out to a library.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/ba7c316474dd6bfc047e5c6dc4dcab39a982caf5.1449965119.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>