Fixed prometheus-cpp patch with forgotted forward declaration

deps/prometheus-cpp/patches/serial_exposer.patch

@@ -1,5 +1,5 @@
 diff --git a/pull/include/prometheus/exposer.h b/pull/include/prometheus/exposer.h
-index c730360..bd06f7c 100644
+index c730360..3d070c5 100644
 --- a/pull/include/prometheus/exposer.h
 +++ b/pull/include/prometheus/exposer.h
 @@ -5,6 +5,7 @@
@@ -7,13 +7,21 @@ index c730360..bd06f7c 100644
  #include <string>
  #include <vector>
 +#include <functional>
- 
+
  #include "prometheus/collectable.h"
  #include "prometheus/detail/pull_export.h"
-@@ -34,4 +35,18 @@ class PROMETHEUS_CPP_PULL_EXPORT Exposer {
+@@ -16,6 +17,7 @@ namespace prometheus {
+
+ namespace detail {
+ class MetricsHandler;
++class SerialMetricsHandler;
+ }  // namespace detail
+
+ class PROMETHEUS_CPP_PULL_EXPORT Exposer {
+@@ -34,4 +36,18 @@ class PROMETHEUS_CPP_PULL_EXPORT Exposer {
    std::string uri_;
  };
- 
+
 +class SerialExposer {
 +public:
 +	explicit SerialExposer(const std::function<void()> update_callback = std::function<void()> {});