Move to pkg-config

Hyprland has recently changed the way plugins recognize the hyprland
headers, moving from directly including the paths with -I option to
automatically generating the cflags with `pkg-config --cflags hyprland`.

This change means that compilation attempts using the direct include
option will start to fail with newer versions of hyprland, which now
uses /usr/local/include for it's headers after `make pluginenv`.

This commit migrates the original system to respect this change,
modifying Makefile, and with it, the include paths for hyprland headers,
which are now prefixed with hyprland/.

include/globals.hpp

@@ -1,5 +1,5 @@
 #pragma once
 
-#include <src/plugins/PluginAPI.hpp>
+#include <hyprland/src/plugins/PluginAPI.hpp>
 
 inline HANDLE PHANDLE = nullptr;