glsl: Rename class variable_entry to ir_variable_refcount_entry.

glsl: Rename class variable_entry to ir_variable_refcount_entry.

Exporting a publicly visible class with a generic name like
"variable_entry" via ir_variable_refcount.h is kind of mean.

Many IR transformers would like to define their own "variable_entry"
class.  If they accidentally include this header, the compiler/linker
may get confused and try to instantiate the wrong variable_entry class,
leading to bizarre runtime crashes.

The hope is that renaming this one will allow .cpp files to safely
declare and use their own file-scope "variable_entry" classes.

This avoids crashes caused by converting src/glsl to automake.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-and-tested-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>