glsl: use ptrdiff_t cast to silence g++ sign warning

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/link_uniforms.cpp

@@ -749,7 +749,7 @@ link_update_uniform_buffer_variables(struct gl_shader *shader)
                if (end == NULL)
                   continue;
 
-               if (l != (end - begin))
+               if ((ptrdiff_t) l != (end - begin))
                   continue;
 
                if (strncmp(var->name, begin, l) == 0) {