common: instantiate strict_si_cast<long> not strict_si_cast<int64_t>

common: instantiate strict_si_cast<long> not strict_si_cast<int64_t>

this fixes the build on armf.

on 32bit platforms, cstdint is very likely to

 typedef long long int int64_t;

this results in compilation error like

 `common/strtol.cc:190:75: error: duplicate explicit instantiation of 'T
 strict_si_cast(const char, std::string) [with T = long long int;
 std::string = std::basic_string]'

 [-fpermissive]
 template int64_t strict_si_cast(const char *str, std::string *err);
 ^`

we can address this by instantiate the primitive type of `long long`
instead of `in64_t`.

Fixes: http://tracker.ceph.com/issues/16398
Signed-off-by: Kefu Chai <kchai@redhat.com>