Don't force installation of static libgamemodeauto

Defining the library with `library` rather than `both_libraries` allows the user
to choose which type they want to install using `-Ddefault_library`.

Closes: https://github.com/FeralInteractive/gamemode/issues/287

lib/meson.build

@@ -25,7 +25,7 @@ gamemode_headers_includes = [
 ]
 
 # Small library to automatically use gamemode
-libgamemodeauto = both_libraries(
+libgamemodeauto = library(
     'gamemodeauto',
     sources: [
         'client_loader.c',