glsl: define YY_NO_INPUT to prevent unused symbol warnings

Otherwise clang warns:

glsl/glsl_lexer.cpp:3507:16: warning: function 'yyinput' is not needed
and will not be emitted [-Wunneeded-internal-declaration]
    static int yyinput (yyscan_t yyscanner)
               ^

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/compiler/glsl/glsl_lexer.ll

@@ -34,6 +34,7 @@ static int classify_identifier(struct _mesa_glsl_parse_state *, const char *);
 #define YY_NO_UNISTD_H
 #endif
 
+#define YY_NO_INPUT
 #define YY_USER_ACTION						\
    do {								\
       yylloc->first_column = yycolumn + 1;			\