build: Don't duplicate Libs in Libs.private in pkg-config file

pkg-config already prepends Libs to Libs.private when you specify
--static so there's no need to duplicate them. Most other projects
don't do this.

configure

@@ -25717,7 +25717,7 @@ else
   SDL_RLD_FLAGS=""
 fi
 
-SDL_STATIC_LIBS="$SDL_LIBS $EXTRA_LDFLAGS"
+SDL_STATIC_LIBS="$EXTRA_LDFLAGS"