Win32: Make thread functions backend independent

This is work towards allowing the Null backend to build for Windows
using the Win32 threading API, without the Win32 backend being enabled.

src/win32_thread.c

@@ -43,8 +43,7 @@ GLFWbool _glfwPlatformCreateTls(_GLFWtls* tls)
     tls->win32.index = TlsAlloc();
     if (tls->win32.index == TLS_OUT_OF_INDEXES)
     {
-        _glfwInputErrorWin32(GLFW_PLATFORM_ERROR,
-                             "Win32: Failed to allocate TLS index");
+        _glfwInputError(GLFW_PLATFORM_ERROR, "Win32: Failed to allocate TLS index");
         return GLFW_FALSE;
     }