glsl/pp: Add forward decls to silence gcc warnings.

src/glsl/pp/sl_pp_macro.h

@@ -31,6 +31,9 @@
 #include "sl_pp_token.h"
 
 
+struct sl_pp_context;
+struct sl_pp_process_state;
+
 struct sl_pp_macro_formal_arg {
    int name;
    struct sl_pp_macro_formal_arg *next;

src/glsl/pp/sl_pp_token.h

@@ -29,6 +29,8 @@
 #define SL_PP_TOKEN_H
 
 
+struct sl_pp_context;
+
 enum sl_pp_token {
    SL_PP_WHITESPACE,
    SL_PP_NEWLINE,