Fix memory leak in mono/mini/aot-compiler.c

Free GString export_symbols that allocates
in add_native_to_managed_wrappers with a
minimum size of 16 bytes.

Signed-off-by: Aleksandr Dovydenkov <asd@altlinux.org>
Found by Linux Verification Center (linuxtesting.org) with SVACE.

Author: dovydenkovas

src/mono/mono/mini/aot-compiler.c

@@ -5570,6 +5570,8 @@ MONO_RESTORE_WARNING
 		fprintf (export_symbols_outfile, "%s", export_symbols_out);
 		g_free (export_symbols_out);
 		fclose (export_symbols_outfile);
+	} else {
+		g_string_free(export_symbols, TRUE);
 	}
 }