Add missing format string in g_print

This fixes build failure due to compiler warning:

playerctl-cli.c: In function ‘main’:
playerctl-cli.c:90:5: warning: format not a string literal and no format arguments [-Wformat-security]
     g_print(g_option_context_get_help(context, TRUE, NULL));
     ^
playerctl-cli.c:161:5: warning: format not a string literal and no format arguments [-Wformat-security]
     g_print(g_option_context_get_help(context, TRUE, NULL));
     ^
Merge pull request #6 from matrach/patch-1

Add missing format string in g_print