glcpp: Reword diagnostic for #elif with no expression

Rather than telling the user what to fix, the standard convention is to
describe what the detected problem is. With this change, test
081-elif-without-expression now passes.

src/glsl/glcpp/glcpp-parse.c

@@ -1950,7 +1950,7 @@ yyreduce:
 		if (parser->skip_stack &&
 		    parser->skip_stack->type == SKIP_TO_ELSE)
 		{
-			glcpp_error(& (yylsp[(1) - (2)]), parser, "#elif needs an expression");
+			glcpp_error(& (yylsp[(1) - (2)]), parser, "#elif with no expression");
 		}
 		else
 		{

src/glsl/glcpp/glcpp-parse.y

@@ -276,7 +276,7 @@ control_line:
 		if (parser->skip_stack &&
 		    parser->skip_stack->type == SKIP_TO_ELSE)
 		{
-			glcpp_error(& @1, parser, "#elif needs an expression");
+			glcpp_error(& @1, parser, "#elif with no expression");
 		}
 		else
 		{

src/glsl/glcpp/tests/081-elif-without-expression.c.expected

@@ -3,4 +3,3 @@
 
 
 
-