Make timer init part of platform API

This is part of runtime platform selection because the null backend
needs the timer implementation for the current OS.

src/null_init.c

@@ -38,7 +38,6 @@
 
 int _glfwPlatformInit(void)
 {
-    _glfwInitTimerPOSIX();
     _glfwPollMonitorsNull();
 
     return GLFW_TRUE;