GH-47742: [C++][CI] Silence Valgrind leak on protobuf initialization

Author: pitrou

cpp/valgrind.supp

@@ -82,3 +82,15 @@
     fun:curl_share_init
     fun:*Azure*CurlConnection*
 }
+{
+    <protobuf>:leak on library initialization
+    Memcheck:Leak
+    ...
+    fun:*google*protobuf*AddDescriptors*
+}
+{
+    <protobuf>:leak on library initialization
+    Memcheck:Leak
+    ...
+    fun:*google*protobuf*RegisterGeneratedFile*
+}