Changed cmake to properly use pkg-config to get gtk-layer-shell

Due to how gtk-layer-shell installs itself, you may need to use
```sh
PKGPKG_CONFIG_PATH=/usr/local/lib64/pkgconfig make -C build/debug
```
to build.

CMakeLists.txt

@@ -44,10 +44,11 @@ add_library(albinos::uvw ALIAS uvw)
 ###
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(GTK REQUIRED gtk+-3.0)
+pkg_check_modules(GTK REQUIRED gtk-layer-shell-0)
 
 add_executable(albinos-service ${SOURCES_SERVICE})
 add_executable(albinos-gui ${SOURCES_GUI})
-target_link_libraries(albinos-gui gtk-layer-shell)
+target_link_libraries(albinos-gui GTK::gtk-layer-shell-0)
 target_link_libraries(albinos-gui ${GTK_LIBRARIES})
 target_include_directories(albinos-gui PUBLIC ${GTK_INCLUDE_DIRS})
 target_compile_options(albinos-gui PUBLIC ${GTK_CFLAGS_OTHER})

README.md

@@ -14,6 +14,13 @@ cd -
 ```
 The compilation may take a while.
 
+Due to how gtk-layer-shell installs itself, you may need to use
+```sh
+PKGPKG_CONFIG_PATH=/usr/local/lib64/pkgconfig make
+```
+instead of just `make` to build.
+
+
 ## Running
 
 To run the service, use: