MINOR: filters: Export the name of known filters

It could be useful to know if some filter is declared on a proxy or if it is
enabled on a stream.
diff --git a/src/flt_http_comp.c b/src/flt_http_comp.c
index 0d3e52b..2d1a7fd 100644
--- a/src/flt_http_comp.c
+++ b/src/flt_http_comp.c
@@ -31,7 +31,7 @@
 #include <proto/sample.h>
 #include <proto/stream.h>
 
-static const char *http_comp_flt_id = "compression filter";
+const char *http_comp_flt_id = "compression filter";
 
 struct flt_ops comp_ops;