glsl/glcpp: Allow single-line comments immediately after #define

We were already correctly supporting single-line comments in case like:

	#define FOO bar // comment here...

The new support added here is simply for the none-too-useful:

	#define // comment instead of macro name

With this commit, this line will now give the expected "#define without
macro name" error message instead of the lexer just going off into the
weeds.

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 */
-"//"[^\r\n]* {
+<INITIAL,DEFINE>"//"[^\r\n]* {
 }
 
 	/* Multi-line comments */