Update 'fake webcam streams ubuntu18'

fake-webcam-streams-ubuntu18.md

@@ -15,6 +15,18 @@ make install
 ```
 sudo depmod -a
 ```
+
+If you faced error for linux headers we should install them manually
+for ubuntu:
+```
+apt-get install linux-headers-$(uname -r)
+```
+for Debian:
+```
+apt-get update
+apt-get install linux-image-amd64 linux-headers-amd64
+```
+
 ### Add Kernel Modules
 ```
 sudo modprobe v4l2loopback card_label="My Fake Webcam" exclusive_caps=1