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-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