glsl: empty declarations should be valid

glsl: empty declarations should be valid

Unlike C++, empty declarations such as

  float;

should be valid.  The spec is not explicit about this actually.

Some apps that generate their shader sources may rely on this.  This was
noted when porting one of them to Linux from Windows.

Reviewed-by: Chad Versace <chad@chad-versace.us>
Note: this is a candidate for the 7.11 branch.