GameScope: Remove trailing whitespace from GAMESCOPE_ARGS string

Now that GAMESCOPE_ARGS is not guaranteed to have any values,
GAMESCOPE_ARGS can be blank when appending flags, so we should
remove trailing whitespaces.

This should be purely cosmetic.

steamtinkerlaunch

@@ -7,7 +7,7 @@
 PREFIX="/usr"
 PROGNAME="SteamTinkerLaunch"
 NICEPROGNAME="Steam Tinker Launch"
-PROGVERS="v14.0.20240829-3 (gamescope-use-getgamescopearg-for-gsshwres-gsintres)"
+PROGVERS="v14.0.20240831-1 (gamescope-use-getgamescopearg-for-gsshwres-gsintres)"
 PROGCMD="${0##*/}"
 PROGINTERNALPROTNAME="Proton-stl"
 SHOSTL="stl"
@@ -11985,7 +11985,8 @@ function GameScopeGui {
 						if [ "$GSHDLS" == "TRUE" ]                                      ; then  GAMESCOPE_ARGS="${GAMESCOPE_ARGS} --headless"; fi
 						### ADVANCED OPTIONS END ###
 
-						GAMESCOPE_ARGS="${GAMESCOPE_ARGS} --"
+						# Remove trailing whitespace and append '--'
+						GAMESCOPE_ARGS="${GAMESCOPE_ARGS# } --"
 
 						writelog "INFO" "${FUNCNAME[0]} - Saving configured GAMESCOPE_ARGS '$GAMESCOPE_ARGS' into '$STLGAMECFG'"
 						touch "$FUPDATE"