MINOR: pattern: replace struct pattern with struct sample

This change is pretty minor. Struct pattern is only used for
pattern_process() now so changing it to use the common type is
quite obvious. It's worth noting that the last argument of
pattern_process() is never used so the function is self-sufficient.

Note that pattern_process() does not initialize the pattern at all
before calling fetch->process(), and that minimal initialization
will be required when we later change the argument for the sample.
diff --git a/include/proto/pattern.h b/include/proto/pattern.h
index 8a896ff..5a9f944 100644
--- a/include/proto/pattern.h
+++ b/include/proto/pattern.h
@@ -26,9 +26,9 @@
 #include <types/stick_table.h>
 
 struct pattern_expr *pattern_parse_expr(char **str, int *idx, char *err, int err_size);
-struct pattern *pattern_process(struct proxy *px, struct session *l4,
-                                void *l7, int dir, struct pattern_expr *expr,
-                                struct pattern *p);
+struct sample *pattern_process(struct proxy *px, struct session *l4,
+                               void *l7, int dir, struct pattern_expr *expr,
+                               struct sample *p);
 void pattern_register_fetches(struct pattern_fetch_kw_list *psl);
 void pattern_register_convs(struct pattern_conv_kw_list *psl);
 #endif