syslog_sink.h: add <array> include (gcc 4.7 compatibility)

The syslog_sink uses the std::array type but does not include it.

Fixes the following error when compiling with GCC 4.7:

    g++ example.cpp -o example -march=native -Wall -Wshadow -Wextra -pedantic -std=c++11 -pthread -Wl,--no-as-needed  -I../include  -O3 -flto
    In file included from ../include/spdlog/details/spdlog_impl.h:33:0,
                     from ../include/spdlog/spdlog.h:143,
                     from example.cpp:29:
    ../include/spdlog/details/../sinks/syslog_sink.h:83:25: error: field ‘_priorities’ has incomplete type
    ../include/spdlog/details/../sinks/syslog_sink.h: In constructor ‘spdlog::sinks::syslog_sink::syslog_sink(const string&, int, int)’:
    ../include/spdlog/details/../sinks/syslog_sink.h:53:9: error: ‘_priorities’ was not declared in this scope
    ../include/spdlog/details/../sinks/syslog_sink.h: In member function ‘int spdlog::sinks::syslog_sink::syslog_prio_from_level(const spdlog::details::log_msg&) const’:
    ../include/spdlog/details/../sinks/syslog_sink.h:92:16: error: ‘_priorities’ was not declared in this scope
    ../include/spdlog/details/../sinks/syslog_sink.h:93:5: warning: control reaches end of non-void function [-Wreturn-type]
    make: *** [example] Error 1

With this change spdlog is compatible to GCC >=4.7.
(at least the examples compile and do what I expect them to do)