perf python: Initialize 'page_size' variable

The commit 0c1fe6b2

:

 'perf tools: Have the page size value available for all tools'

Broke the python binding because the global variable 'page_size' is
initialized on the main() routine, that is not called when using
just the python binding, causing evlist.mmap() to fail because it
expects that variable to be initialized to the system's page size.

Fix it by initializing it on the binding init routine.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-vrvp3azmbfzexnpmkhmvtzzc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>