glsl/glcpp: Fix NULL directives when followed by a single-line comment

This is the fix for the following line:

	#  // comment to ignore here

According to the translation-phase rules, the comment should be removed before
the preprocessor looks to interpret the null directive.

So in our implementation we must explicitly look for single-line comments in
the <HASH> start condition as well.

This commit fixes the following Khronos GLES3 CTS tests:

	null_directive_vertex
	null_directive_fragment

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

src/glsl/glcpp/glcpp-lex.l

@@ -250,7 +250,7 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 	}
 
 	/* Single-line comments */
-<INITIAL,DEFINE>"//"[^\r\n]* {
+<INITIAL,DEFINE,HASH>"//"[^\r\n]* {
 }
 
 	/* Multi-line comments */