glcpp: Recognize plain "//" as a comment.

Found in glsl-orangebook-ch06-bump.(frag|vert).  This was resulting in
the comments getting passed through to the main compiler's lexer.
diff --git a/glcpp/glcpp-lex.l b/glcpp/glcpp-lex.l
index 0dea995..fabe756 100644
--- a/glcpp/glcpp-lex.l
+++ b/glcpp/glcpp-lex.l
@@ -60,7 +60,7 @@
 %%
 
 	/* Single-line comments */
-"//"[^\n]+\n {
+"//"[^\n]*\n {
 	yylineno++;
 	yycolumn = 0;
 	return NEWLINE;