Wrap public declarations with G_BEGIN_DECLS/G_END_DECLS

Glib release 2.67.3 started conditionally using some C++ code[1] in the
public headers and it's no longer legal to include "glib.h" or anything
that includes it transitively from `extern "C"` block.

The solution recommended by the glib developers is to enclose all
declarations in a header within G_BEGIN_DECLS and G_END_DECLS as shown
in gobject developer manual[2].

[1] https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1715
[2] https://developer.gnome.org/gobject/stable/howto-gobject.html#howto-gobject-header

lib/searpc-client.h

@@ -15,6 +15,8 @@
 #define DFT_DOMAIN g_quark_from_string(G_LOG_DOMAIN)
 #endif
 
+G_BEGIN_DECLS
+
 typedef char *(*TransportCB)(void *arg, const gchar *fcall_str,
                              size_t fcall_len, size_t *ret_len);
 
@@ -137,5 +139,6 @@ searpc_client_generic_callback (char *retstr, size_t len,
 #define TRANSPORT_ERROR  "Transport Error"
 #define TRANSPORT_ERROR_CODE 500
 
+G_END_DECLS
 
 #endif

lib/searpc-named-pipe-transport.h

@@ -16,6 +16,8 @@
 #include <windows.h>
 #endif
 
+G_BEGIN_DECLS
+
 // Implementatin of a searpc transport based on named pipe. It uses unix domain
 // sockets on linux/osx, and named pipes on windows.
 //
@@ -72,4 +74,6 @@ int searpc_named_pipe_client_connect(SearpcNamedPipeClient *client);
 LIBSEARPC_API
 void searpc_free_client_with_pipe_transport (SearpcClient *client);
 
+G_END_DECLS
+
 #endif // SEARPC_NAMED_PIPE_TRANSPORT_H

lib/searpc-server.h

@@ -15,6 +15,8 @@
 #define DFT_DOMAIN g_quark_from_string(G_LOG_DOMAIN)
 #endif
 
+G_BEGIN_DECLS
+
 typedef gchar* (*SearpcMarshalFunc) (void *func, json_t *param_array,
     gsize *ret_len);
 typedef void (*RegisterMarshalFunc) (void);
@@ -134,4 +136,6 @@ gchar *searpc_server_call_function (const char *service,
 LIBSEARPC_API
 char* searpc_compute_signature (const gchar *ret_type, int pnum, ...);
 
+G_END_DECLS
+
 #endif

lib/searpc-utils.h

@@ -10,6 +10,8 @@
 
 #define SEARPC_JSON_DOMAIN g_quark_from_string("SEARPC_JSON")
 
+G_BEGIN_DECLS
+
 typedef enum {
     SEARPC_JSON_ERROR_LOAD,
     SEARPC_JSON_ERROR_PACK,
@@ -80,3 +82,5 @@ inline static void json_array_add_json_or_null_element (json_t *array, const jso
         json_array_append_new (array, json_null ());
     }
 }
+
+G_END_DECLS