Improve Makefile

[davxy]

Fix build warnings

Add only cpp sources to SOURCE_FILES to remove annoying warning about #pragma once

❯ make all
g++ -shared -fPIC --no-gnu-unique src/Flash.cpp src/IFocusAnimation.cpp src/main.cpp src/Shrink.cpp src/Flash.hpp src/IFocusAnimation.hpp src/Shrink.hpp  -o hyprfocus.so -g `pkg-config --cflags pixman-1 libdrm hyprland` -std=c++2b
src/Flash.hpp:1:9: warning: #pragma once in main file
    1 | #pragma once
      |         ^~~~
src/IFocusAnimation.hpp:1:9: warning: #pragma once in main file
    1 | #pragma once
      |         ^~~~
src/Shrink.hpp:1:9: warning: #pragma once in main file
    1 | #pragma once
      |         ^~~~

Optimize artifact

• Add -O2 optimization to compiler options
• Strip hyprfocus.so after build (size comparison: 8MB vs 120kB)