glsl: Separate varying linking code to its own file.

linker.cpp is getting pretty big, and we're about to add even more
varying packing code, so split out the linker code that concerns
varyings to its own file.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/linker.h

@@ -91,4 +91,13 @@ private:
    void recursion(const glsl_type *t, char **name, size_t name_length);
 };
 
+void
+linker_error(gl_shader_program *prog, const char *fmt, ...);
+
+void
+linker_warning(gl_shader_program *prog, const char *fmt, ...);
+
+unsigned
+count_attribute_slots(const glsl_type *t);
+
 #endif /* GLSL_LINKER_H */