Use GMutex instead of GStaticMutex

Because GStaticMutex is deprecated since GLib 2.32.0.

This change suppresses warnings as followings:

    milter-memory-profile.c: In function 'profiler_log':
    milter-memory-profile.c:61:5: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
         g_static_mutex_lock(&profile_mutex);
         ^
    milter-memory-profile.c:84:5: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
         g_static_mutex_unlock(&profile_mutex);
         ^
    milter-memory-profile.c: In function 'milter_memory_profile_report':
    milter-memory-profile.c:135:5: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
         g_static_mutex_lock(&profile_mutex);
         ^
    milter-memory-profile.c:140:5: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
         g_static_mutex_unlock(&profile_mutex);
         ^
    milter-memory-profile.c: In function 'milter_memory_profile_get_data':
    milter-memory-profile.c:168:5: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
         g_static_mutex_lock(&profile_mutex);
         ^
    milter-memory-profile.c:172:5: warning: 'g_static_mutex_get_mutex_impl' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:149): Use 'GMutex' instead [-Wdeprecated-declarations]
         g_static_mutex_unlock(&profile_mutex);
         ^

GitHub: see #32