Remove unnecessary -I$(top_srcdir) when it's an individual gem

Author: aycabta

ext/readline/extconf.rb

@@ -109,5 +109,4 @@ def readline.have_type(type)
   $defs << "-Drl_hook_func_t=Function"
 end
 
-$INCFLAGS << " -I$(top_srcdir)"
 create_makefile("readline")